Apparently they saved a lot of money filming Dr. Horrible by using this guy's house. He was on Monster House a few years back and got them to turn his place into a mad scientist's lab, all they did was add a computer. They put him in the party scene at the end as a thank-you, I'm pretty sure he was the Purple Pimp.
I've been trying to figure out if they filmed in a real laundromat which one it might be - one screencap I saw had the numbers 1732 and 1734 in big numbers above the doors.
Only thing Joss did wrong was make it exclusive to iTunes; if Zune Marketplace carried it I'd buy it in a heartbeat.