I recently gave the Spirit Airlines low fare calendar a try while organizing a short vacation, and I must admit that I was taken away. I first assumed it would just show costs at random, but it clearly showed the cheapest days to fly. I was able to get a round-trip ticket for much less than I was expecting. The only restriction is that your trip dates need to be a bit variable.
The savings are significant if you can make the adjustment by a day or two. Clearly a useful tool for budget travelers like me!