SelfControl is a free and open-source application for Mac OS X that lets you block your own access to distracting websites, your mail servers, or anything else on the Internet. Just set a period of time to block for, add sites to your blacklist, and click “Start.” Until that timer expires, you will be unable to access those sites–even if you restart your computer or delete the app.
You can get an equivalent for the PC or Android phone called Self Control for Study. It works on the same principles but doesn’t look as nice as the the Mac OS X version. There have also been a few reviews that mentioned a few bugs in earlier version but that seem to have been rectified in the latest release.
What’s good about them: They both allow you to set your own limits and make it impossible to cheat yourself.
What’s not so good about them: They will not work alone in getting you the grades you want. You will still have to put in the hard work!