Handheld computers have a variety of ways of accessing the Internet. Fancier handhelds have built in WiFi for accessing the Web wirelessly. For handhelds without this wireless networking feature, there's FlingIt (or it's cousin, Plucker), which allows you to send selected web pages from your desktop computer to your handheld for viewing. (Internet Explorer for Windows Mobile also has this ability). Once on the handheld, the page can be beamed to other handhelds.
The challenge is finding pages that look good on a handheld's smaller screen. Pages that are mostly text, have a limited number of images, and simple formatting are easiest to read. There are some sites that are specially designed for handheld screens, like PalmInfoCenter, Yahoo, and PDA Portal. Another trick to find pages that suitable for handhelds is to do a Google search and include "text only" (type the quotes, too) in your search. For example, for shark research into Google's search field: sharks "text only".
You might be familiar with the free online encyclopedia that can be edited by anyone, Wikipedia. It can be a useful site for students to do research. Now there's a handheld-friendly version of Wikipedia called Wapipedia. It takes the content from Wikipedia and puts it in format that is appropriate for devices with small screens .Whether you're using a wireless network, FlingIt, or some other method of viewing web pages on your handheld, Wapipedia will prove to be extremely useful in classrooms.
For more information on Wikipedia, read Andy Carvin's blog post, Turning Wikipedia into an Asset for Schools.