Kajaani Kombat
Castle Combat
UFO 2000
FreeCiv
Battle for Wesnoth
netris
0verkill
...a whole lot of MMORPGs...
You can find many of these on happypenguin.org, unfortunately there's no 'multpilayer' category, but it's easy to find them with keywords.
I'd post links for the games I mentioned above, but I'm lazy and they're trivial to google.
The first five are available for Linux, Windows and other OSes, dunno about the last two, as they use text console...
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