I use to use overlib an it does have a lot of functionality, but I recently started using jtip for the ajax request functionality which worked out great. You don't have to use ajax with jtip. Also there is a tweaked version that better handles positioning at the corner or bottom of pages etc. Granted the other tooltip plugins are great for simple tips.
I only mention jtip because some folks looking at the jtip plugin may not realize this updated version is also available: http://www.learningjquery.com/2006/10/updated-plugin-jtip I just wish there was a way to put a delay on jtip so when its open, you can either delay it or keep it open unless someone closes it so if you have clickable hyperlinks inside the tip one can click on them.
