Looking to rent San Fran apartment

I am looking to rent an apartment in San Francisco for 3 weeks in September, anyone know of some good websites or better again does anyone know of one available. Suitable for 2 and close to the tourist things - Nob Hill, Russian Hill Union Square or The Wharf. Thanks

CraigsList springs to mind

try the real estate section of the San Francisco Chronicle web site, www.sfgate.com. they usually have listings on there for holiday rentals

The steinhart comes highly recommended