Windows Vista has gadgets, Mac OS X has widgets, KDE users have plasmoids, but what if you’re an Ubuntu user? Don’t worry, you have a number of opinions, you just have to install them yourself.
gDesklets - Project Website
The first option I looked at was gDesklets. For Ubuntu users, they are available from the standard repositories, so head over to Add/Remove and search for “gdesklets.” After installing gDesklets, you will want to add the command “gdesklets shell” to your session (System > Preferences > Sessions > Add). Now either restart your computer or open them once manually by going to Applications > Accessories > gDesklets. This will bring up the “shell,” from which you can add widgets. A huge number can be installed just by navigating through the sidebar at the right or you can look on the gDesklets site. You can install downloaded widgets by going to File > Install Package.
In order to bring widgets to the front of your applications you can (by default) press Shift + F12. If Compiz Fusion is not enabled, there is a box around widget of the desktop background that looks really ugly. Turn Compiz Fusion on and it looks right.
Screenlets - Project Website
Screenlets, like gDesklets, can be installed through Add/Remove if your on Intrepid. If your still using Hardy Heron, it is still in the repositories, but you have to go to Synaptic. To make it start up when you boot, launch Screenlets from either Applications > Accessories > Screenlets (usually Intrepid) or System > Preferences > Screenlets (usually Hardy), then check the box saying “Auto start on login.” From this same window, you can add the widgets that come with Screenlets. You can also download them from the Screenlets site. Then just click the install button and select the package. You should also be able to install Google Gadgets with Screenlets, although I have not tested this.
If you want to be able to bring your widgets to the front, you can follow this guide which uses a Compiz Fusion plugin.
Conclusion
These two options are quite similar in many ways.and there are also other options avaliable, but these seem to be the most common. For me, I am going to stick to Screenlets, since it has a nicer interface and the Compiz Fusion widget layer integration is really nice. In the end, though, it is really up to personal choice. Since neither is difficult to install, I recommend you try out both before settling on whichever one suits your needs best. Now, off to find some more widgets!