For the last few months, my MacBook's fan seems to stay on way longer than necessary. I can close the MacBook's cover and still the fan is going like crazy. I'm not really sure what the problem is and for better or worse, the symptoms are not so severe to make me spend that much time troubleshooting until I find it.
This evening though, it seemed like it went on for more than normal. I tried quitting out of all my open programs (and it's true that there were quite a few) but it still wouldn't stop. A bit desperate, I opened up the Activity Monitor (it's in the Utilities folder) and clicked on "Disk Activity" in the header to the lower half of the window:
And I could at least see that I wasn't crazy and that the disk was being used and therefore something must still be going on.
That's when I realized that Photoshop was still open. I control-clicked its icon in the Dock and saw that the "Application was not responding" (or however they word it). Once I force quit (another option in that same menu), the Activity Monitor showed that the activity was lessening, and the fan finally stopped.
The other thing that I found interesting was the little CPU Usage window. To make it appear, choose Window > CPU Usage:
You can have it float above your other windows as you work in order to see which programs are taxing your CPU most. The idea is that if you feel like things are going slower than normal (or slower than you'd like), you might be able to pinpoint which program is the culprit. Or in my case, if the fan starts going crazy, and it corresponds with increased CPU activity, maybe I can figure out why.