A lot of us use the Hilton and/or Starwood American Express Optima cards. Both cards are free, although the Starwood card is now becoming free for the first year and $30 a year thereafter, still cheaper than most others out there. A benefit to having the Starwood card is that after acquiring 20,000 points, you can transfer the points to many major airlines and Starwood will throw in another 5,000 points. For transfers less than that the transfer rate is 1-1.

There is currently a long thread on the Starwood board about the new version they've just introduced. That would tell you quite a bit more.