I've been doing a lot of vacation planning lately. At one point in time I was planning because I actually intended to take a vacation. But then I found out that my preferred vacation partner prefers to spend his time remodeling kitchens instead of traveling with me (that's not totally true - it's just that he will be remodeling for the next four billion months, it seems). I thought about traveling alone but I did that for 3 months last year and it wasn't really my favorite thing in the world. So now I'm just planning vacations that I won't ever take.
In the process I've come across some great travel resources:
I don't know that I'd actually make a purchase through this site, but there's a great option (under Deals) to search multiple destinations and multiple dates. Just choose a departure city and month - it will tell you the least expensive places and dates to travel.
Zuji is based outside of the US and it will, therefore, allow you to do a 'flexible date' search for international flights (US-based sites aren't allowed to do this because of some fuel surcharge issue that I don't really understand).
Mobissimo searches flights from a whole slew of travel websites all at once. It's absolute brilliance. (And as a rational economic agent, it's probably the closest I'll ever get to the intangible ideal of a perfectly competitive market equilibrium based on perfect information - which makes me a happy girl.)