Valve are offering a free taste of Team Fortress 2 all weekend for those few that still haven't made the plunge. All you need is a Steam account - check out the Gold Rush site
for some more info. If you haven't played it yet, now's the best time to give it a crack as it's probably the closest you'll get to a demo version.
There's also a small TF2 update with a few minor fixes that has been rolled out as well; the details are available on the Steam website