Think: I have already talked about think elsewhere. Think enables the user to focus on just one application, seeing nothing more. It is like the blinders of a horse to keep you from distracting.
LeechBlock: A FireFox addon, which allows you to block certain sites at certain times of the week, or after a predetermined amount of time has been wasted there. A generic *.com *.org *.net filter, workdays from 9 to 13, 15 to 19 could be enough to keep you from wasting time, couldn't it? But it can be customized to allow for a little every hour (I let 5 minutes each hour in this span... you may need to look for something)
emacs: As emacs is almost everything, it also enhances my productivity. Usually I am always in emacs, doing whatever I am working on (looking for references, writing something in LaTeX).
pageAddict: As Julio suggests in his comment, pageAddict can be a nice addition. It tracks your internet usage, which you can then view graphically (i.e. 17 minutes of Facebook, 10 minutes of Google, 40 minutes of Wikipedia...). I have tried it, it works pretty well, but I guess I already know where my time gets wasted... If you don't, give it a try.