I had posted this earlier under United:

I don't have a comprehensive list or comparison of such agencies.

One such site is
http://www.cendant.com/ctg/cgi-bin/TravelersAdvantage/

They have an annual membership fee, but will give you a hotel certificate good for a free night at select discount hotels. I think that this is the same outfit which offers CitiTravel for CitiBank customers. They also have a money back policy on their annual fee.

Another site is www.travelersnet.com

A good search engine may point to other sites which offer rebates.