Sometimes walking around outside in the area you want to live in works. Sometimes asking friends on or off of Facebook works.
Sometimes there are city-specific rental sites. If you're in San Francisco, this might help: http://rentsfnow.com
I would definitely suggest just going out for a drive in the area you want to live and looking for places that way. Unfortunately, I found that this technique doesn't work nearly as well when you're trying to rent a house.
The issue is that the data are typically fragmented and often proprietary and aggregation websites are focused on a revenue stream from the listing agent rather than the renter.
Craigslist is still ultimately king but if you want to be comprehensive, you might need to check a few different places. Local newspaper websites are good too.