I find the links toolbar in Firefox very useful. I add all my commonly-used links, grouped into folders. E.g. under "Tools" I have "application" sites like online banking. Under "News" I have Google Reader and a few news sites I like to keep up with. Under "Discuss" I have a few forums. That's not the trick, though - here's the trick:
A few days ago I moved all forum and news-type sites to a new folder...
Seriously, it works. Now instead of unconsciously opening a news site when I'm thinking about something and blowing half an hour (that I can't afford), I'm very aware of what that folder represents! I'm treating sites in that folder far more gingerly. Every time I wander over to get a dose of news I see "Waste Time" and I slink back to doing some work!