If you are a knitter, or a crocheter (is that what people who crochet call themselves?) or a spinner or a weaver, or anyone who likes to play with yarn in any way, Ravelry is hugely addictive. I have to admit that that is my most recent excuse for being a slacker blogger...(I have so many excuses).
Seriously though, if you take Facebook and Wikipedia and Yarn and mix them together, you get Ravelry. If you bother to read my blog and you like to play with yarn, go there now and sign up...the current waiting time for invite once you sign up is only 3 days...I spend way too much time there these days checking out new patterns ideas and looking at other peoples projects.