We love Hotels.com! They have a rewards program that gives you 1 free night for every 10 you stay. That promotion can be combined with their other promos that seem to be ever improving!
Like this one: Stay at least 3 nights, at a total of $450 ($150 per night) and get a FREE Amazon Kindle! Enter code BOOKWORM by 6/27 (must stay by 7/31) at checkout. That's it!
They also have a 2 night promo (with no spending minimum!) where you get a $25 gas card. Enter coupon code ROADTRIP by 6/27 (must stay by 7/31)