I typically use kayak.com/explore to find the lowest previously-found fares for each major city from my origin, and then use matrix.itasoftware.com to do a flexible search between the cities, and nearby cities, to identify all cheap travel dates.

Matrix.itasoftware.com lets you search for flexible dates within a month's span (i.e., "January 5 and a month later," or 1/5/12-2/4/12) and for trips with variations of up to seven days (i.e., "0-7 days," "15-22 days," or "6-8 days"). It also lets you specify airlines and fare codes and either demand or allow for certain connections. So, if you know a certain routing you want on a certain carrier at a certain minimum fare code (good for meeting upgrade requirements or planning for mileage-run promos), it's very useful.