It's free for both. The Nvidia offer also has a portal demo and Peggle Extreme, ATI only has HL2:DM and Lost Coast (which is just a graphical demo pretty much)
No you don't need HL2 to play it, but once you have HL2 you can download and play any source mod, which opens up a lot more options for game night. (I'm looking at you, Hidden: Source)

