For those who aren't familiar with Bitcoin, I'll try sum it up. Bitcoin is a peer-to-peer electronic currency based off encryption to avoid having a central authority (that is, there's no one person in charge of it who can be corrupted). Because there's no central authority mandating it's use, there's nothing to give it value either, besides the voluntary participation from it's users.
That might sound like a really bad thing, but look at it from this perspective: there's nothing that gives the USD value anymore either (it's not tied to gold in any way shape or form, for example) except a) the threat of violence from the US military and b) people who know they can turn around and use it to get something else they want already. Bitcoin only possesses one of these things, though in a much weaker incarnation (it's not always apparent you'll be able to get what you want without first converting it to your local currency first).
So anyway, it's imaginary money that you can earn by cooking your graphics card, or you can accept in exchange for goods and services. Just as a silly little experiment, MumbleDog accepted Bitcoin in exchange for a couple server rentals (we figured worst case scenario we'd be no worse off than the few servers we sponsored to groups) which we then did quite well off of. The first thing I "bought" with Bitcoin is a new graphics card - though I didn't really buy it with Bitcoin, I sold the coins and bought it with the proceeds. I would have gone with an Nvidia card, but they don't work so hot for mining Bitcoins so I went with AMD instead. The card has since paid for itself, even after accounting for electricity costs. At the moment because profitability is so low and the temperatures outside are so high, I've stopped running it so hard, but it's still essentially a free graphics card.
Speaking of the graphics card, it came with a free copy of DiRT3, which at the time I had little interest in playing on account of I'm a Gran Turismo fanboy. I sold it, for about $25 in Bitcoins. Recently I had an opportunity to buy a copy back for about a quarter of that price, and the "Get your Free Copy" postcard that came with my GPU has been haunting me, so I bought DiRT3 back with Bitcoins, only to realize I'm terrible at it.
I also picked up a gift code for Minecraft for my brother in law's birthday, well under what it would have cost me to sell the Bitcoins and outright buy the code. It was worth it because first of all I got to play it a bit, but that's an addiction waiting to happen so I've since stopped. Second, my brother in law hasn't shut the fuck up about Minecraft since he first saw it - he would spend hours playing the "classic" (free) version, or watching Let's Play videos on YouTube or whatever. So it was basically the perfect gift.
Speaking of things my brother in law won't shut the fuck up about, I also picked up a copy of Terraria which I let him play, being a mistake because he spent the next five hours talking to me about it like a coked-up horse racing announcer. Again, I paid well under what would have been retail and it worked out great, but again I will probably not touch it because like an alien abduction or Minecraft, I have a tendency to lose several hours before regaining consciousness of my surroundings and wondering where the hell I am and where everyone else went.
Just by being involved in the shitstorm that is MtGox, I managed to score a free Yubikey 2-factor authentication device. Of course right now it only works with MtGox, and despite the fact that I don't really care for MtGox's site the extra security of 2-factor auth has meant if I do want to sell some Bitcoin, that's where I head. If Bitcoin goes under or I lose interest in it, then after cleaning out my MtGox account I can flash the Yubikey and use it for SSH or something.
Finally, I've essentially bought a paid back-link, by being a non-anonymous donor to Bitcoin-Charity and donating to Doctors without Borders. I would have donated to the Humane Society (because dogs are awesome) but the site operator said that once any particular charity hit a full Bitcoin in donations he'd make the first cash donation transaction, and I wanted to bump that process along.
Update (2011-08-31): I picked up S.T.A.L.K.E.R. - Call of Pripyat for about 1/5th of retail price. I thought it'd be more like a photo-realistic Borderlands, but honestly I'm having a really hard time figuring out what the heck is going on with it, so I shelved it until I'm done playing DiRT3.