Second Life - Free Basic Account
I don’t normally get into these things, but Second Life is running a “Second Anniversary Giveaway” promo (only through July 13th!) that allows you to get a Basic Account for free. These normally cost $9.95, so Free is Better. 
Sign up, download the SL client software (Windows only, as far as I know), and try it out. It’s quite cool… I’m away from home until Friday, and my current laptop can’t quite handle the graphics load of SL, but you might run into me “in-world” some time. My SL name is Plato Dana (explanations by request only).
Update 1: Second Life is not Windows-only - they’ve also got a Mac OS X version.
Update 2: You’ll have to enter your credit card info when you register, but don’t be alarmed - it won’t be billed as long as you select the “Basic” account type (and the promo hasn’t ended).
Update 3: TODAY (JULY 13th) IS THE LAST DAY OF THE GIVEAWAY. Don’t wait - join now! It’s certainly not for everyone, but it’s definitely worth checking out for free.