For straight up looping and storing backing tracks the Jamman series is great. Buy used for a better deal. On the flip side the Zoom G3 has a great 40 sec looper synced to it's internal drum machine. It's the only looper I know of with a progress bar on the display as a visual aid. And it's the cheapest looper with a synchronized rhythm machine.
I used the G3 looper on the following songs in my soundcloud link...