All you need is a free Steam account and access to a computer with an NVIDIA card.
You can definately play Portal (The First Slice), but it also gave me HL2: Deathmatch, Peggle Extreme, and HL2: Lost Coast. I possibly get the HL2 games because I already have HL2 and HL2: E1.
My birthday isn't even until tommorow :-)
Thanks to Jon for the tip-off.