T R E M U L O U S is the only game that I really waste my time with these days. It's a standalone FPS+RTS combination (think Savage, but without the "commander" aspect) based on a modified Q3 engine. It's a very standard aliens vs. human marines setting, but both teams are radically different, and having a good builder can make or break your team.
The learning curve can be steep, but it's worth checking out if your the type that enjoys a well-thought out FPS.
Also, it's only officially supported for Linux and Windows, though an
unnofficial Mac version can be found, provided you're diligent enough to get to it.