Hi Omar,
What kind of 'widgets' are you looking for? I'm not qauite sure how to answer your question, so I'll throw a few ideas into the pot, and you can get back to me if I haven't hit the mark.
If you want a set of pre-built desktop-style widgets (trees, tables, etc.) then qooxdoo (
http://qooxdoo.sourceforge.net) provide a fairly comprehensive suite. Rico (
http://openrico.org) also have several pre-built widgets, notably a data grid.
The Scriptaculous library is starting to produce widgets (
http://script.aculo.us). It also has several very nice 'Effects' - visual transitions that can be used when building widgets, particularly good for notifying the user.
These are all JavaScript libraries - I'm assuming that you want to hand-code yourclient, if you're asking on this forum. There are server-side frameworks that will generate the client code for you, e.g. Backbase (
http://www.backbsase.com : commercial with free version) and some of the
Java Server Faces implementations e.g. Oracle's ADF. You won't get quite the same flexibility of layout/design with these, of course.
Hope that helps,
Dave