Valve has added Multicore rendering support to Team Fortress 2 with varying degrees of success.  There is a benchmark on Steam Forums.

Added Multicore Rendering

• This initial release is aimed at testing compatibility, so the option is OFF by default.  To turn it on, go to the Options->Video->Advanced dialog, and check the "Multicore Rendering" option

Other Changes

• Several performance improvements to decals and client bone/flex setup
• A variety of alt-tab and mode switch fixes
• Improvements to the way the engine initializes surround sound, fixing some specific hardware casesFixed a crash on exit in Vista 64