Hey, I want to buy a one way ticket from Canada to the Netherlands. I'm only staying there for a couple days before heading south to Belgium, France and Spain. Is it okay to buy a one way ticket or do I need to have a return? Are there any Visa requirements?
Thanks in advance!

Well you will get varying answers on the one way question aniaib. Some people will tell you that you may be refused by the airline unless you buy a return ticket or can show some other form of 'proof of onward travel'. That can easily be done if you are planning on buying a rail pass for example before leaving Canada or have a bus ticket from Amsterdam to Belgium as another example. It will also depend on what passport you are travelling on. If you have a passport from an EU country then no questions will be asked at all.
It is also quite possible that even travelling on a Canadian Passport you will not be asked anything on landing and can fly on a one way ticket. The issue is the POSSIBILITY of being stopped from boarding by the airline.
Regarding visa requirements, that of course depends on what passport you are travelling on. If it is a Canadian passport you do not need a visa for any of the countries you list but you are limited to a maximum stay in all of them combined of 90 in any 180 calendar day period.

If you don't have an European nationality and can prove it (e.g., a passport), you are required to have a ticket back home to Canada. Canadians are exempt for tourist purposes trips up to 90 days, consecutive or not, within any 180-day window.
No you don't have to have a ticket back to Canada europegrad. Here goes this BS again on the subject.
What the AIRLINE is afraid of is if you are refused entry into a country. So they cover their ass by insisting on a return ticket. If you are refused entry the airline can be fined and has to fly you back to your point of origin.
What Immigration MAY ask you for is proof of onward travel. A return ticket provides that proof but so does any ticket out of the country. ie. a bus/train ticket to X will do. But that is ONLY if you are asked. For most tourists getting off a plane in Europe the question is never asked.
What Immigration is looking for is people who may intend to stay in the country illegally and probably work illegally. So it is only when they are suspicious and start asking questions that this even comes up. Being able to answer their questions with reasonable responses can be enough. ie. 'I am going to spend 6 weeks travelling in Europe and will fly home from either Spain or Portugal depending on how my time and money hold out.' The individual Immigration Officer has the authority to allow you entry into the country or deny you entry. Proof of onward travel is one way to deny you if they are suspicious.
This topic gets way too much misinformation written about it.
can show some other form of 'proof of onward travel'. That can easily be done if you are planning on buying a rail pass for example before leaving Canada or have a bus ticket from Amsterdam to Belgium as another example.
A return ticket provides that proof but so does any ticket out of the country. ie. a bus/train ticket to X will do.
yes, proof of onward travel is required BUT not just onward travel out of the Netherlands but out of the Schengen zone. A ticket from NL to Belgium will NOT do. A ticket from AMsterdam to London will.
Why don;t you buy a return ticket? you'd better have a good explanation in case you do get challenged by either airline or Immigration. And what is your nationality?

Assuming you are Canadian:
http://www.worldtravelguide.net/netherlands/passport-visa - normally quite reliable - says you DO need a return ticket.
An onward ticket would do. Whether out of Schengen or just out of the Netherlands I don't know
An onward ticket would do. Whether out of Schengen or just out of the Netherlands I don't know
Immigration requirements are managed by the Schengen treaty- not by individual Schengen member states. Hence, if the requirement is to have a ticket for onward travel, it will be out fo the Schengen zone. That makes perfect sense given that there are no routine border formalities within the Schengen zone so leaving NL for Belgium would be impossible to enforce or verify.
There is a difference between the legal requirements you MAY be asked to show compliance with and what actually happens at the Immigration Desk on arrival. Hands up how many people arriving at any European Airport from Canada, on a Canadian passport, have been asked to show a return ticket or proof of onward travel. When is the last time an Immigration Officer asked to see anything other than your passport?
I think you will find very few are ever asked. Again, it is usually the Airline who insists on this to cove their ass. Immigration rarely asks a typical tourist.
I'll repeat that I think this issue is vastly over-blown here on the TT. Nine times out of ten at least you can arrive on a one way ticket with no problem. Take for example Canadian Affair who fly between Canada and the UK every day. They structure their pricing based on one way tickets. Anyone can buy a one way and they will not insist you buy a return UNLESS they have reason to be concerned such as your nationality. That is, If you have a Canadian passport they are unlikely to be concerned but if you have an Albanian passport (example) they may well refuse to board you with a one way ticket. The fact is not everyone is treated the same. You can go through their booking process right up to the enter the credit card number to see for yourself if what I am saying is correct. http://www.canadianaffair.ca/
There are many circumstances under which you may not know when you plan to return and obviously there has to be a way to deal with those circumstances. For example, I recently flew to the UK to attend a funeral. As I was booking my flight from Canada as soon as possible without first knowing the funeral date, I wanted to just get there right away and then book my return a few days after the funeral. I booked a one way, flew, got asked no questions by anyone and booked my return when I was ready to return. If I had been asked by Immigration why I had no return ticket I would have explained the circumstances and my plans. I am 100% confident they would have let me into the country.
So while booking a return or onward travel doesn't hurt, it is not necessarily mandatory to do so if your circumstances dictate that it is not practical. Immigration Officers aren't dumb, they know what they are looking for (illegal overstayers/workers) and what is reasonable.
Aubo23, what a website says you must do and what you actually MUST do are not one and the same thing. As someone I know used to always say about newspapers, 'paper prints anything'. Similarly, websites can print whatever they want, it doesn't make it so.
travelerinstyle, you're right that the majority of US/Canadian passport holders do not get any grief from Immigration in Europe. However, there's no need to get all haughty about it. The requirement does exist but, as you said, Immigration officers use discretion and are far less likely to give a Canadian retired couple hassle than an African male in his 20s.
However, OP didn't provide us with his/her nationality. (s)he also did not explain why they would be travelling on a one-way ticket. Are they flying out of Paris a week later? Are they overlanding to Japan to swim across the Pacific back to Canada? or hoping to find some work in Spain (one of the FAQs on this board)
So unless OP provides some more background all of it is just speculation and all we can do is just state the rules.
BTW I (and many others) have been asked by airline staff to show proof of onward travel and/or right of entry documents (e.g. visas). Airlines do check these things. The fact that you can buy a one-way ticket from Canadianaffair on-line proves nothing. Air France will happily sell me a return ticket to Russia but will still refuse to let me board if I show up without the appropriate visa (speaking from experience here).
Also, I don;t know what the entry requirements are for Canadians travelling to the UK but they might be different than those for Canadians travelling to Schengenland. You still share a Head of State after all so maybe the UK is more lenient towards Canadians- don't know but don;t assume that your particular experience with one country applies to others.

Aubo23, what a website says you must do and what you actually MUST do are not one and the same thing. As someone I know used to always say about newspapers, 'paper prints anything'. Similarly, websites can print whatever they want, it doesn't make it so
True that countries do not always enforce to the letter all their immigration requirements. But if they choose to do so you are sunk.
And your example of flying to the UK isn't really helpful - the UK doesn't have an onward ticket requirement. So no one is going to insist you have one. That doesn't mean to say that other countries in Europe operate the same rule Different country = different rules.