We’ve all wondered whether we should buy our plane tickets right now or wait for a few days to see if the prices drop. Using Farecast you can determine if another week of waiting will do you good. And, for $3 Farecast will guarantee your original plane ticket price, so you don’t end up getting a more expensive ticket if you wait based on their recommendation.
The only downside is that Farecast only works for US domestic flights. Perhaps they’ll add international destinations as their popularity (and revenue) increases.
I’ve posted before the best time in general to buy plane tickets (Wednesday nights between 11pm and 1am). For international flights check FareCompare.