I've sold a lot of my old stuff (mostly video games, textbooks when my semester ends) on eBay for nice profits. A month of easy work resulted in $70, which isn't a big amount, but it's quite a bit of extra spending money for me. People have also offered to pay me for helping to set up/style their websites but I usually don't ever charge. I enjoy doing it and it's usually help for friends. If it was a big job, though, I'd consider it.
Yes, just during the Steam Summer sale, i sold trading cards and made enough cash to buy myself Oblivion. Twas a good weekend
Usually I end up selling stuff on eBay like trading cards or other things I don't need anymore. The bulk of my earnings come from art commissions on deviantART and another art gallery.