Gary's website:

www.freefrequentflyermiles.com

has always been my favorite (of course the focus is in airline and hotel cards), but I always check the insanely zealous folks at:

www.fatwallet.com

forums before I do anything. Many of these cards also can be applied for through the new websites like:

www.cardbenefit.com

where you might get bonus sign-up cash for your CC application. Make sure the offer is the same as what's on Gary's website first though. Sometimes the offers through these websites give 5 - 10k less miles.