Mow a lawn
Ebay (Also a good way to clean out your room)
Loose change
Borrow 50 cents from 20 friends (If you have that many)
Yard Sale
Pawn Shop
Have a friend chip in a bit and let them play a bit
Charge people for pencils, gum, pens, paper etc. (I know in my school, people constantly ask for those things. If you charge a small amount for them, you can make quite a bit of money in a short amount of time)
I'm not going to post one here because I'm pretty sure it's bannable, but you could give one of those survey sites a try.
That's about all I can think of at the moment.