Screenlets are themed mini-applications programmed in Python. They are comparable to the widgets in OSX and Windows. Screenlets are drawn with vector graphics and nice transparency effects, and therefore need a compositing window manager like Compiz.
This post will guide you through installing Screenlets and setting them up with Compiz Fusion to work similarly to widgets in Apple’s OSX. Screenlets are available in both Ubuntu Feisty and Edgy. Add the line to your /etc/apt/sources.list file that corresponds to your version of Ubuntu.