I personally only have experience with Hilton, but if your new to this forum, check out the thread below discussing the Hilton Gold Upgrade and 50,000 point promo. If you sign up with Hilton HHonors and register with promo code "EWRDC" by June 1, 2001, you receive an automatic upgrade to Hilton Gold status through April 2003. It doesn't help you much at Hampton, but at Hiltons, Embassy Suites, and Doubletree hotels, it usually either gives you a room upgrade, or the free breakfast that you are looking for (Hampton gives a free breakfast to all guests).

Also, if you make 4 stays within 90 days of registering, you earn 50,000 hilton points. Check it out:
http://www.flyertalk.com/forum/Forum57/HTML/001664.html