By and large, we here at Hackcollege think that doing your college work on a computer is generally preferable to older more traditional methods. This of course isn't always the case, but bear with me. The problem is, if you spend most of your study time on the computer you'll be coping with all sorts of distractions.
If you get easily sidetracked from your school work, and are the kind of person who would be willing to cripple your own computer to narrow your focus, well then SelfControl may be just the Mac application for you. The premise is simple: you tell SelfControl what websites and email accounts you want disabled on your computer, and for how long. For example, I could have used SelfControl at work the other day to keep myself from filling out the pre-order application for the iPhone 4 seven million times. I didn't, but I probably would have gotten a lot more work done if I had if I had.
If you find yourselves two hours deep into a writing session and suddenly get desperate to watch a Youtube video, you're just totally out of luck. Restarting the computer, or even deleting the program won't make a difference. Obviously there are better ways to stay focused (true self-discipline being chief among them), but for the student on a deadline with some attention span concerns, SelfControl may do the trick.