This calculator lets you calculate training paces for various workouts, and also shows equivalent race performances. For instance, if you ran 20 minute 5k, the calculator will predict your 3k, 10k, and marathon time. Check it out - it's fun and has a good scientific basis by exercise physiologist Jack Daniels. More info/instructions on how to use the calculator here (and link to the original calculator here).
This is a good basic chart for pacing track workouts, and 3k and 5k pacing.