Entering Sudan is usually straightforward for visitors carrying a passport with a validity of at least one year. Visas are available on arrival for most nationalities, provided visitors have an entry permit arranged via a local tour operator or a hotel.
It's strictly forbidden to take alcohol into Sudan.
Everyone, except Egyptians, needs a visa, and getting one could be the worst part of your trip.
Some embassies are easier to deal with regarding visas than others, and in all cases a transit visa (which gives you up to a fortnight to transit the country) is easier to get than a month-long tourist visa. Note that if there is evidence of travel to Israel in your passport you will be denied a visa. Currently Aswan (Egypt) remains the easiest places to get a visa; they are normally issued in a couple of days or even less there. A tourist visa is very hard to get in Addis Ababa (Ethiopia), but transit visas are possible. In Europe, the embassy in Vienna is reportedly the easiest place to get a tourist visa. Expect all this information to change constantly.
For a tourist visa, it helps a lot to let a Sudanese tour operator arrange it. Most of the time they will get you a counter visa: you email them a copy of the first page of your passport and they arrange everything at the Ministry of Interior in Khartoum; after this they email you an entry permit, which you'll need to board the plane to Sudan; on arrival you show your entry permit at immigration, pay US$100 (US$150 for American citizens) and collect the visa. Sudanese tour operators typically charge US$150 per person for the visa service. If you are lucky, it can take as little as three days (but 10 days sounds more realistic).
If the tour companies give you the runaround (some may be reluctant to offer this service if you don't book a tour with them), some hotels in Khartoum can also arrange an entry permit. The Bougainvilla Guesthouse and Acropole Hotel are very helpful in this regard. They may ask you to book a few nights on top of the service fees.
Note that you'll need at least two blank pages in your passport.
You have to register within three days of arrival in Khartoum, Port Sudan, Gallabat or Wadi Halfa. In Khartoum, go to the Aliens Registration Office; the process costs S£250. You need one photo and photocopies of your passport and visa (there's a photocopier in the building) and a letter from a sponsor in Sudan; your hotel will normally act as your sponsor and provide you with the required letter. Even cheap hotels should be able to do this, although you might have to go and collect the required form from the office for them to fill in. If for some reason your hotel can't or won't do this, then local tour companies may accept to complete all the registration formalities for you for about S£150. If you're travelling with a tour company they will take care of this for you. If doing it all independently allow several hours and a headache.
There is also another registration office at the airport. Technically this office is only for emergency cases and shouldn't be relied on, but on Fridays, when the main office is closed, you can do it here.
If you registered on entry at a land border, you need to do it again in Khartoum, but you don't have to pay again. In all towns where you overnight you will need to register with the police – this is free, and it's a straightforward process; most hotels or guesthouses can do it on your behalf.
The Sudanese authorities have always been renowned for their paranoia about foreigners nosing about their country. All travel outside Khartoum requires a travel permit. Take one photo and a copy of your passport and visa to the Ministry of Tourism and Wildlife in the Riyadh area south of the city centre. A permit covering everywhere you intend to visit (except the no-go zones) can be issued on the spot. This permit is a combined travel and photograph permit. Carry dozens of photocopies of this permit along with copies of your passport and visa to give to police at checkpoints and when checking in at hotels.
Visa extensions are issued at the Aliens Registration Office in Khartoum. You need one photo and varying amounts of money and patience to get your extra 30 days.
Egypt This consulate is not the most organised place – it's easier to get a tourist visa on arrival (which most but not all nationalities can do), especially if you're flying, or at the border (US$20) if you're travelling from Wadi Halfa.
Ethiopia One-month visas cost S£250 and require two photos. You can pick your visa up the same day.
Saudi Arabia Visa applications are handled by travel agencies (many of which surround the embassy), which can get you a transit visa in two days. You need a visa to a neighbouring country (normally Jordan), two photos and a mandatory insurance (from US$32). Visas are not issued during the hajj and nor are they issued to unmarried women under 40 unless they are accompanied by their husband or brother (and can prove it).
South Sudan Not the most helpful of embassies and more than a little vague about what is required in order to obtain a tourist visa! What you will need though is a hotel reservation/letter of invitation, a letter or invitation from your embassy, US$100 and two passport photos. Visas take one or two days to issue.