Farecast.com is probably the coolest travel site I've seen yet. It tells you when airfare is going to be cheapest and whether you should go ahead and buy your tickets now for that trip to Las Vegas in November or if you should wait a while for optimum pricing. It can also tell you which departure days (or which destination) will get you the best prices, if your schedule is flexible. When the beta first launched they only had Seattle and Boston as departure cities, so it wasn't really useful yet. Now they've just added Denver, so they have 55 or so cities. All the destinations I've cared about recently are in there.
Check it out if you might be travelling anytime soon.