Wi-fi is widespread in hotels and cafes (you may need to ask for the code), and can be found for free in many tourist offices and other public places. Open your wireless device and you'll often find paid wi-fi access from KPN (www.kpn.com), the national phone company, and other providers such as T-Mobile (www.t-mobile.nl).
Internet cafes are few and far between, although storefronts selling cheap phone cards with a few terminals can be found near train stations in larger towns. Many libraries, tourist offices, coffeeshops and hotels provide internet terminals (sometimes free). Expect to pay anything from €2 to €4 per hour.
If you're surfing on a smartphone that works in the Netherlands, beware of potentially astronomical roaming charges (around €1 per minute depending on your provider).