I like the smartsound stuff alot - easy to tailor a track to just the right length. They have a cinema group of tracks that has some pretty good stuff.
I use shockwave-sound.com. They are cool because they provide comping versions of most of their tracks so you can see if it works before shelling out any money. And, their tracks are fairly reasonably priced.
I've been using Digital Juice. Prices are good (somethings always on sale) the tracks are pretty good and you can hear them all on the web site so if you know what your getting.