The BOSS JS-5 JamStation is a device that provides digital instrument accompaniments that can be programmed for various styles and 128 instruments. Today, there are software products that do this for the iPad for a fraction of the cost, but the quality of the sequencing, the presets, and the selection of instruments is very good. The only drawback is some limitations in the ability to customize a song. For example, there is a jazz template for a 5/4 tune, but you can't make up a tune in other odd meters, like 7/4, from scratch. As you can see from the buttons there are three times as many rock templates available as jazz. You can save your own songs in the User area or on an SD storage card. No batteries--it has to be plugged in with its own power supply. It has mono and stereo outputs, MIDI out, and headphone jack.
Here's an example of a song I recorded with it a few years ago. It's Unit 7, a classic Wes Montgomery tune. It's an MP3, about 4.4MB. I recorded this on a Zoom 4-track digital recorder. First I recorded the JamStation track to the Zoom, then overdubbed the guitar track.
As with most of these kinds of complicated products, it comes with a thick manual that is difficult to use. I wrote a guide called The JS-5 Jamstation for Geniuses (Musicians Are No Dummies!) (PDF document) for people who are not dummies, but can't get much out of the manual.