AppHarbor. I can't wait till I've got some spare time to go play with what they're doing. I suspect they're going to be ridiculously big even if .NET isn't very popular in these parts.
There are some things I don't like, I'm used to dedicated servers and I miss things like scanning the eventlog looking for where my stuff screwed up (could handle it on the front end but meh, the eventlog's so convenient!), but overall it's quite awesome.
I really hope they manage to package their stuff up so it can be installed on other servers.