Shortly before starting my job at Google, I had invested a little time into writing a Chrome extension called Crackbook. It helps against wasting time on the web by delaying access to selected sites (see this XKCD cartoon for an illustration, and check out the hint too). Because access is just delayed rather than blocked, you are less likely to just disable the tool after a while, which is exactly what happened to the other tools I tried to use.
Having found a little free time in the evenings, I have just updated the extension. If you use Chrome, please try it and give feedback!
The extension also collects anonymous statistics on what domains people tend to mark as junk and how often the domains are hit. Some very basic results are available at crackbook.info. Now that a fair amount of data has been collected, a deeper statistical inspection is in the plans. Stay tuned.