True, wikipedia! Didn't add my list but a whole lot of time is spent there but i'd hardly consider it as a waste because you're probably learning a lot whilst there. It does tend to lure you in more and more, so much so that you end up reading other articles and not what you intended to actually read.
Ah, totally forgot about Wikipedia...I've probably spent hundreds of hours reading random entries on there. I don't consider it a waste of time either, since you usually come back knowing one or two new things.
I have no time to waste but find time to utilize it in some of the respected webmaster forums like dp, webmaster-talk etc.