One of my favorite applets is a system monitor which shows you CPU, Memory, and Network Usage over time. I feel this is key to monitoring overall system health. XFCE4 has conky, but I really wasn't happy with it.
After much searching I finally figured out how to setup the gnome system monitor applet on my panel:
sudo apt-get update
sudo apt-get install gnome-system-monitor gnome-panel
Although you may not need to, I had to reboot before I could see the new applets.
Add a panel, Xfapplet.
Move it to where you'll want system monitor to be first.
Rigth click on the panel -> proporites -> System Monitor.
Now you can right click on system monitor and add whatever features you wish.