UT2004
it's like $10, but if you buy it in the Unreal anthology it's $20, and includes three other games
if you buy the pack on steam though, it's $40, because it contains UT3 as well
which is okay. UT3 is fun too
the problem with buying the Unreal anthology is that I'm not sure if you can get it digitally anywhere except steam
you'd have to order it
also, the demo for UT2004 is free, and you can play MP with it
very limited with your maps in the demo though