Travelzoo has a top 20 newsletter every week, and they have all the best travel deals from flights, hotel and vacation deals. I got a RT ticket on British Airways to London from San Francisco for $300 with tax through one of their deals.
I have also found that Travelocity has some good deals, just use the "dates don't matter" option and it will show you the best prices and when those are available.
Flights.com is a plane ticket consolidator, meaning they buy bulk tickets and sell them at a discounted rate. Cheaptickets.com is pretty good for flights as well.
For planning I love Lonelyplanet's Bulletin Board called the Thorn Tree. If you are thinking about Europe, Eurotrip is a great site for that, though geared towards backpacking.