Video: The Raspberry Pi usually runs cool enough that you don't need a fan, but if you're overclocking it or running something particularly intense, it's helpful. Don't go hunting for some specialised fan though, as YouTuber NetHacker points out, any old computer fan will work.
NetHacker pulled out a fan from a junk computer, plugged it into the Pi's GPIO, and got it working instantly. If you have an old computer sitting around, it probably has a fan that will do the same job just fine. If not, you can usually snag one for a couple of bucks.