Can't say enough good about Turbosquid.
For me the main appeal in terms of your question is that you can be reasonably sure that the models you find there that are free are actually INTENDED to be free by the creators...on less..reputable sites you can probably download models for free, but the creator didn't intend them to be distributed that way.
Also, while I think there might be a few that ride that thin line, there are not likely to be many that truly infringe on IP - - you won't find an X-wing, Mickey Mouse, etc.
There is a LOT of variation in quality, but there are quite a few solid models for free, and a lot of the low poly ones I will often use as starting points rather than just popping them into a render. A well designed low poly item can be quickly made 'all your own', giving you a cool model and experience with the modeling tools.
Also, you might want to look at (depending on what kind of scenes you do) E-interiors.
http://www.e-interiors.net/index.php?page=index&left=frameleft&up=frameup&down=home¯ocat=home&la...