http://www.kayak.com - good site for finding deals.
http://www.tripadvisor.com/ - This site will give you a heads up on what to expect at almost any resort or location. Reading individuals reviews will give you a pretty good idea if it was written by a prissy that broke a nail or a down to earth expectaions person that truely had an awful experience.
http://www.selloffvacations.com/
http://travelocity.lmdeals.com/ - There are lots of "Last Minute" sites out there and good deals to be had if your okay with waiting and then taking whatever pops up. If your only real priorities are, Ocean, Beach, hot and all inclusive this might be the way to go... Besides Mexico (except perhaps for the Baha California area) is overrated imho... the Mexican people seem to have come to despise tourists and you will feel that resentment... I'll never go back to the Myan rivera (Cancun, Tulum area) but I'd give the Baha California a chance.