Donating your used car to a non-profit organization is one of those ways. It doesn't matter what condition your car is in, there are always places that are willing to take used cars -- even cars that don't run can be donated for a worthy cause. So if you have an old junker that's taking up space in your garage (or worse! your lawn!), consider giving it to one of the fine institutions below.
Here's a list of places that you can donate your cars, whether they run or not!
Wheels for Wishes
Helping Hands of America
Habitat For Humanity
P.S. To the people who are Googling for "donate junk car for cash" -- you are missing the point! I think you should be looking up "SELL junk car for cash." By its very definition, the word "donate" means that you are giving something away, not selling it for a profit. Perhaps you should try actually donating something sometime, and see how that feels.