On my own time, I've done some stuff.

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A port of xscreensaver-3.22 by JWZ.
Download: Win32 screensaver
It's probably pretty broken. I haven't touched it for a couple of years. It uses OpenGL though.
Source code:
Source code notes: notes.txt

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I also ported XSoldier to Win32.
The original one is here: XSoldier

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I also made a port of X11R6.4 server to Win32 because the ones out there either cost money or are too old.
X.org

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I ported netris to windows for nic.
Download: netris.zip

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I like to port things to windows. I'm not saying that windows is better than any other GUI, but it's just fun and easy to do.