I use two:
-Multimeter (does what the name implies)
-Clear Clock (nice simple digital clock, with a transparent background)
I'd use an RSS gadget if I could find one that didn't irritate me.
And, occasionally I use the slide show gadget when I need to have the desktop beautified while I work.
But to be honest, gadgets are still kind heavy on memory for what they do. On my setup anyway, your experience may vary.