My brain craves novelty. And without it, unless I'm incredibly focused and interested in something, I need something for my novelty fix. The Internet helps with that.

Sometimes it's work related. Sometimes it's not. I measure my guilt towards the latter with how much more or less productive I am than some of my coworkers. As long as I exceed said individual's work output greatly, I feel less like I'm stealing company resources. Plus, if I happen to be screwing off as programming on personal projects, I can think of it as industry research. Or sharpening my tools.