Another one I submitted, it was uploaded last week. xtrans.tcz.
This combines xcompmgr and transset-df. xcompmgr is an X composite manager and transset-df is a tweaked version of the transset program. They require a video driver that supports the COMPOSITE extension. I use nvidia-glx. The ATI and the xf86 drivers have not been tested.
Instructions for enabling the basics really quickly and testing it by enabling real time terminal window transparency (I use this a lot) are in the .info file.
It's a gimmick, but a fun one. Besides a transparent xterm (another one I submitted to the repository!), it allows conky to have a transparent background too if you like to have it running on your desktop.
xcompmgr is basically a lightweight alternative to compiz.