Unfortunately many of the applets do not compile. At least not on my Kubuntu 8.04 system with KDE 4.1b2. If you want to try out a few, checkout one of these: calendar charselect darkstat embed-win keyboardLeds previewer timer toggle-compositing wifi-signal-strength.
I especially like wifi-signal-strenght: Easy but nice and rather pretty. Get it with
svn co svn://anonsvn.kde.org/home/kde/trunk/playground/base/plasma/applets/wifi-signal-strength
And compile it with:
cmake .. -DCMAKE_INSTALL_PREFIX=/usr/lib/kde4/
You may have to replace with PREFIX with the one from your distribution.