I used Stax during beta, and it's nice, but it's no near the Heroku ease of use (puls it's little helper Nezumi http://nezumiapp.com/ when on the way). With very small restrictions, one can install anything on Stax, so it's very flexible, but just not that easy to use as Heroku'.
another, https://www.playapps.net/ for play apps., those written using the playframework.