I've had a Flickr account for almost five years now, and aside from providing me with a surprising number of cool contacts and friends, it's also given me a wealth of resources to better my craft. Not only are there incredible forums and discussions about anything and everything under the photography sun, it's a venue for feedback, encouragement and constructive criticism.
From Flickr alone, I've had some crazy opportunities I'd never have gotten otherwise, including having select photos published in one book, two magazines, countless blogs, websites, and my personal favorite - an album cover. Pretty sweet! For $25/year, I'd say that's a bargain!
Just this week I was approached by the marketing manager at Easy Stand to have a photo used in their blog about healthcare reform. It's especially neat when I can be a part of something bigger than my own little photo world and help to visually support someone else's cause. (In this case, I was just glad the blog argued a position I agree with!)
Anyway, if you're a photo buff but not a Flickr member, take a second look and consider joining (membership is free for up to 200 photos!). I'm just sayin'.