I play keys and host a few jam bands. We get some musicians (and especially singers) that don't know where the beginning (or end) of a phrase is and, even though I've set up a drum machine to play a cover song, things don't go as planned. Best thing I've been using so far is an old Korg i3 intelligent arranger (just for the drums). I can select a kit, pattern, tempo, etc, then hit buttons on it to trigger from several intros, verses, fills, endings. So if the sax decides he wants to do another chorus (with a different feel) I just hit another verse loop. If the guitarist is stoned and plays a thirteen bar blues phrase, I can just trigger a fill and we're back in the groove.
Unfortunately, that 20 year old instrument's buttons are getting frazzled and I need to find something else. I can't believe that 20 years of technology hasn't come up with something better - or even similar! I'd like the ability to tweak the patterns and loops (midi?) but it has to be able to perform live with the bands, easily.