Lots of people skip working out when their mood isn't ideal because they don't have the mental energy to switch gears, but the trick lies in finding the right workout to match the mood you're in. For example, some workouts have a calming effect, while others are stimulating. Here's what I recommend to keep you exercising no matter what your state of mind:
 
When You're Angry - do something that involves your mind and keeps you from focusing on what has you angry. Play squash, or take an aerobics class you've never tried. Learning new moves will free your mind from what's been upsetting you.

If You've Got the Blues - If you're not up for the usual high-energy stuff, do some leisure activity you enjoy, such as digging in your garden or walking in a park. View it as mental recreation, not exercise.

If You're Feeling Bored - Being around people is a quick and easy way to beat boredom. Playing a sport with them is even better, try some tennis or golf. Get together with a group that walks or goes for bike rides on a regular basis. Being outside with other people is invigorating and engages your mind.
 
When You're Stressed Out - When your brain is overwhelmed and anxious, you need to turn to a mindless activity to settle it down. Something repetitive such as swimming or walking on a treadmill requires little mental input and is most effective at reducing feelings of stress and increasing calmness.