Bad Mood?

Run.
Take a kickboxing class.
Rockclimb.
Bike.


Absolutely nothing will make you feel better than working out, no matter what kind of funk you're in.
Transfer the anger or frustration you feel at the world into a harder workout. The more upset you are, the harder you can dig into your workout. When you finish, you've gotten a double bonus: you've worked out AND you feel better.

Here's some suggestions for bad-mood workouts:

Intervals on the Treadmill:
Start by warming up at your own easy pace for 5-10 minutes, then alternate 1 minute of sprinting with 2 minutes of jogging or walking. When you sprint, concentrate on using your negative emotions. Repeat for as long as you want, ending with an easy 5-10 minute cooldown.

Spinning:
If your gym has a spin room, it's the perfect place to work out stress. If there's no classes available, grab your iPod and try making up your own routine. After a 2-or-3 song warmup (low resistance, moderate speed), alternate the following with easy song breaks:
-"climb"- choose a 4 or 5 minute song, increase resistance at a constant speed
-"jumps"- at a medium resistance, bike in your seat for an 8-count, then come out of the seat, keeping your butt above the seat, leaning on your elbows for another 8-count, then bike standing up as straight as you can for another 8. Repeat for the whole song.
-"sprints"- at a low resistance, sprint out the chorus of a fast-paced song.

Do a 1-2 song cooldown when you finish, and stretch!