Search giant Yahoo! recently bought Konfabulator and their first order of business was to get rid of the $19.95 charge and start giving it away for free. I've been using the Dashboard for Mac OS X, which is by all accounts copied from Konfabulator, and I think I like Konfabulator better. One reason is that it's available for Windows, too, so I can set up my widgets at work to match my widgets at home and have some sort of continuity. Another reason to prefer Konfabulator is that you can do all sorts of thing with the widget windows. You can make them part of the desktop, you can make them always on top or you can set them so they're only visible when you display the widget layer with Konsposé (press F8 and all the widgets become visible and the background is obscured). I've been trying out widgets, and here are some of my favorites so far:

Radar - Nice, resizable weather maps
systemDashboard - Memory Monitor - Monitor memory and hard drive free space
Multi clock - Who doesn't want an old-timey pocket watch on their desktop?
THCalc - Nothing special, just a calculator

Those, along with the weather forecast and calendar widgets that come pre-installed, are what I'm using now. If you have some other widgets you like, leave a comment and I'll try them out.