Estimated Cost: $0
Despite what you may think, the process of overclocking your CPU is not as daunting as you may think. As long as you've got adequate cooling in your system it's a quick trip into the BIOS to adjust voltage and the front-side bus.
Certain processors are more stable when overclocking than others, but there are plenty of guides online with details on how to handle each chip. It's important to follow instructions very carefully as you can fry your chip.
However, for those willing to take the risk, this is the best way to increase the overall processing speed without buying any new hardware.