nitrous.io provides an interesting service. I have experimented with the free tier and I am considering going on a paid plan. One big plus is the live collaboration support (I have not tried it, just read about it).

One thing that I am not sure about (some clarification would be good): the higher level paid plans are advertised to (soon) have custom domain support. So, in addition to being a development platform, is it also meant for deploying web apps that don't need to scale to large numbers of users?