My favorite and a few of the guys I play with as well, is the "Tempo" app from Frozen Ape. I use it both on my phone and iPad. It is incredibly flexible, and I love that you can set up playlists to make gigging with it incredibly easy.
I want to say it costs $2 in the App Store, something like that. There is a great free two week "bootcamp" on all the ins and outs of using it on fretboardanatomy.com
I used to play at open mikes with the guys from The Practically Hip (canadas #1 tragically hip cover band!). They did it do commandeer some of the wayward jammers. It keeps people in line but it made sure no one was ever really creating a groove because someone was going to be off time slightly eventually, and instead of listening to each other WE OBEYED THE MIGHTY KORG.