eBay
I know a lot of people who hate eBay/PayPal. While I don't love their pricing policies, there is something strangely addicting about selling stuff on eBay. I find that I kind of get on a roll once I start selling. I sell a couple of games or an old DVD player and suddenly I'm looking all over my house thinking "what else can I sell?"
It's especially cool when you find something you own (like an out of print DVD or a duplicate Lego set you picked up on clearance) now selling for 4 or 5 times what you paid for it. Once, while cleaning out an old file cabinet at work, I found a still sealed copy of MS-DOS 3.3. I got out on eBay and was amazed to see what it was selling for as a "computer antique". One man's trash truly is another man's treasure.