Here are the ones I know of...
http://www.online-sweepstakes.com/ - AKA OLS. Probably the largest domain for sweepstakes on the web. Members have a variety of options and can even track entered contests. My favorite of the sites thusfar but one thing worth mentioning is that premium members (ones who choose to pay a daily/monthly/yearly fee) get access to the listings of the best (or I guess, most valuable) giveaways and competitions. Free members still have a lot to sift through though.
http://www.slickdeals.net - I <3 this site. If I'm looking to buy -anything- I go there and search first. Their forum has a contests section where several dedicated members submit what they find. It doesn't have quite the level of organization of OLS though.
http://www.sweepsadvantage.com - Seems to have about 1/2 to a 1/3 of what OLS has. Layout is also kind of outdated. Everything is free though, no Premium member stuff.
http://www.grandmajam.com/ - Still perusing the site, nothing to comment on yet.
http://www.contesthound.com/ - Still perusing the site, nothing to comment on yet.
Any I'm missing? :D