I'm sure that most everyone interested in knitting, crocheting, looming, felting, &/or spinning already knows about Ravelry, but I just have to add my plug, here too!
I signed up for an invitation a couple months ago, after hearing about it on a few of the creative podcasts I listen to, and got my invitation in January. Now, to be honest, I like to sign up for everything that remotely interests me, so I can get my username before anyone else gets it. I guess that's kinda geeky of me, but it still annoys me that on any/all Yahoo! application, I have to use my amended username, because someone else got mine! So I promptly went to sign up as soon as I got the invite, but I didn't really check it out much.
I finally took an actual look around a couple weeks ago, since I'd been actively hookin' for a little bit. And I am sooo hooked! The layout is awesome, there are tons of groups, it's easy to navigate, connects easily to other web things, like flickr and blogger, and is a super community. I encourage everyone to go check it out, you might find the perfect group (or twenty) for you.
And I had my first opportunity to tell someone about Ravelry in public, too. I bumped into my friend, Z, handing out pie samples yesterday, and she commented on my hat, and turns out, she's a no pattern crocheter, too! So I had to tell her about Ravelry, of course. Come join us.
BTW, on Ravelry, and most anywhere else ;p I am "seeshells".