My bank and credit union both offer car buying services via TrueCar (which my bank actually owns around 10% of so they really encourage people to use it). In my case I paid around 5% less for my car than the "best" price I was able to get by visiting a dealer and going through the traditional song-and-dance routine. It seems like one of the big benefits is that they make it really easy to check the prices at a bunch of dealers. I don't remember if they actually offered any substantial saving at my local dealer compared to the price I "negotiated", but I was able to find a dealer that was willing to do a cheaper price around 70 miles away.

A bunch in the bay area. I use San Francisco Federal, but they use PatelCo CU's service.