This developer's site has an excellent introduction to Ajax, including demonstrations (with code) of sortable data grids, popup suggest boxes for form entries, and more.
An ideal first read if you want to get into Ajax.
This site, by the author of 'SAMS Teach Yourself Ajax in 10 Minutes', is a repository of code examples, tips and tutorials about Ajax, including some completely-worked projects you can try for yourself. There's also an enthusiastic forum, moderated by the author, where you can ask and answer Ajax questions, post sample code etc. Pay a visit.