The Boss RC-3 Loop Station will do that, it's got 100 or so spots and if you plug in the USB, it just shows up as a drive with 100 folders. Drop your file into whatever number you want and set that preset for one-shot playthough, and it'll just play whatever file is in that folder. I've used it for everything from just having a 808 bass drop to emphasize during a song, to intros, to playing backing tracks, etc. Probably used it more for that sort of thing than as an actual looper. You can use an external footswitch to change the preset as well, although I haven't needed to do that yet.
Actually it might not do MP3s, everything I've always done is .wav, but that's not the end of the world.
If not with its own DI, I run it with my AX8 into the effects return, and blend it in at the end of the signal chain to FOH. If I'm using my guitar with a piezo output I'll run the acoustic signal through the looper as well, so I can have an acoustic loop under the AX8 signal, and still bring in the external sound effects too.
