Compton is a compositor for X, and a fork of xcompmgr-dana.
I was frustrated by the low amount of standalone lightweight compositors. Compton was forked from Dana Jansens' fork of xcompmgr and refactored. I fixed whatever bug I found, and added features I wanted. Things seem stable, but don't quote me on it. I will most likely be actively working on this until I get the features I want. This is also a learning experience for me. That is, I'm partially doing this out of a desire to learn Xlib.
Changes from xcompmgr:
inactive window transparency (specified with -i)
titlebar/frame transparency (specified with -e)
menu transparency (thanks to Dana)
shadows are now enabled for argb windows, e.g. terminals with transparency
removed serverside shadows (and simple compositing) to clean the code, the only option that remains is clientside shadows
The above features give compton a feature set similar to the xfce compositor.
Compton has only been tested with openbox so far, but frame transparency should work with any window manager that properly sets _NET_FRAME_EXTENTS.
Fixes from the original xcompmgr:
fixed a segfault when opening certain window types
fixed a memory leak caused by not freeing up shadows (from the freedesktop repo)