On a deadline? Have a daily word-count commitment you're at risk of breaking? Check out Dr. Wicked's Write or Die. This web-based text program allows you to choose a specific word goal, and prompts you to keep writing until mission accomplished. You can choose different grace periods (how long it will let your fingers stop moving) and types of prompts (from gentle to electric shock). I'm not sure how serious of a tool this is, but it's sure fun to see how writing under the gun effects your style. I used the program to write a first draft of this, and had a blast.