BIAB and RB are about the only game in town for accompaniment generating programs worth their salt for that purpose. There's a couple more much smaller players without the following of the PG stuff.

There's lots of DAW that allow the use of loops and transposition of them as barbarton describes. This is how Garageband, Tracktion, Ableton, ACID, FL Studio and perhaps more operate.

Garageband is a great DAW, with excellent included content (the included amp simulation is really quite good), but it's not an accompaniment generating tool.

-Scott