You need to be aware not only of which program the card is affiliated with, but also which bank issues the card. For example, Chase Bank issues both the United and the Continental cards. Chase (and other banks) will often limit the number of applications you can make in a short time.
The websites mentioned above are good. And you will find threads on FT about many/most credit card offers. Sometimes you will find threads on offers in Miles Buzz, sometimes in a particular airline's forum, sometimes in the credit card forums. Search is your friend.
Unless you are looking at an offer that expires very soon, take the time to see what is out there and what fits your needs the best.