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.
What is the reason to use them vs. Microsoft Azure? (I know very little about .Net, Azure, or AppHarbor, but Microsoft seems to position Azure as the .Net hosting solution). I agree that if that isn't the case, they're the top of this YC batch; otherwise idk.
(Not knowing much about .NET myself, I can only repeat what they said in their pitch). They apparently use a standard stack, whereas Azure runs on a heavily customized stack that requires extra work to make a standard .NET app run on it. Think Django vs. Django on AppEngine.
There's hardly any configuration with AppHarbor. I've launched several projects on it already and it is as easy as Heroku. On the other hand, Azure requires a lot of application customization and configuration. So you can think of AppHarbor as "Azure done right."
So today I actually did go and play with AppHarbor, I managed to deploy a bunch of sites with almost no changes at all to code/config. That's not going to happen on Azure.
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.