There's also HireFire, which has support for scaling all your Heroku process types, not just your web processes: http://hirefire.io/ .
I've used them for a few projects which honestly have never actually needed auto-scaling, but they're very easy to use overall and at $10/month are way underpriced. They do have a slower polling frequency (1 minute instead of 15 seconds) though.
HireFire is fantastic (can't recommend it enough for people using Heroku). We use it at backstitch (http://backstit.ch) and it has saved us a ton of cash early-on.
I've used them for a few projects which honestly have never actually needed auto-scaling, but they're very easy to use overall and at $10/month are way underpriced. They do have a slower polling frequency (1 minute instead of 15 seconds) though.