Do you mean like a looper pedal that loops what you play or a bypass loop? If it's the first option the ditto looper by TC electronics is pretty awesome. Simple and to the point. With up to five min of loop time and you can have up to two layers of overdub. I think it's two. There are quite a few out there. I've heard the boss loop stations are pretty boss...... Hehe....... I also have a hardwire DL8. It's a delay and a looper in one.
If you meant a bypass looper, there's tons of those. I use a 6 channel bypass looper by pedal racks. That's the only one I've used though. It works great for me.