Or a garden hose, bucket of soapy water, and a sponge. At least if you have a house with a garden hose. Many apartment complexes offer the amenity of an area for washing cars though.
Car washes are an order of magnitude more water efficient (enforced by state agencies) than individual home car washing.
I live next to a college. The Sororities will hold car washes. Of course, they ask us to get out of the car first, so that Cheerleading movie w/ Kristen Dundst lied. :-)
Most Canadian cities prohibit washing anything (car, bike, etc) outside with soap unless you're capturing the runoff. Even biodegradable soaps.
Car wash is the only legal way to use soap, for better or worse.