A meme that's been going around blogs and Facebook lately is that it's very easy to throw productivity in the toilet browsing the internet and whatnot at work.
This obviously isn't true when you're excited about what you're working on - when you're in that first 10-20% of the project, for example, when everything's new and you're creating with wild abandon, or when you're learning something new and interesting.
But every project has its 80% perspiration, where it becomes a slog and you just wish it was done and you don't want to fix a network play bug and it's tempting to check facebook during that build and next thing you know you've spent an hour surfing the net even though your build was done 58 minutes ago. The evils of multi-tasking.
I am weak - I succumb to that temptation more than I should - but I can't just unplug the internet, because I need access to engine documentation and articles on math and so on and so forth. I need to block specific troublesome sites. So lately I've been using Stay Focusd to keep myself on target. It only works for Chrome, but they only ask for a $10 donation and it's very fully featured.