There are some great internet filters out there and some are free. The one that usually gets very good reviews is K9. K9 is free both for Windows and Macs. The other good ones are netnanny and safe eyes but those cost money. Always do your research before you buy any of them to see if there is a feature that you need that only some filters have.
The downside of a filter for yourself is that if you need the internet and you have blocked it you will not be able to get on. There is a solution to this: Freedom
If you are having trouble with your internet usage you might want to check out Freedom, which works for both Windows and Macs. I have this program and I find it very useful. You tell them how long you want to not be able to go on the internet and then you will not be able to go on the internet for that time period. In the event of something urgent, you can always restart your computer and get back on the internet. I have used this many times when I should be starting and I find myself surfing or blogging. The only downside is that it used to be free but now costs $10 but if you are someone who does have trouble controlling your internet time I think it is a wise investment.