It's not that similar to OF (even remotely), but there's also 'savage' (or 'SFE' as it's now become).
It's a quirky RTS/FPS/RPG which is unlike anything else I've played. I'm not sure how active it is today...
http://www.newerth.com/smf/index.php/topic,259.0.html
[Savage 2 win/linux is rather heavily delayed, but hopefully will be released/beta'ed this year somewhen]
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