You can also do javascript with it. I can't speak to it personally but friends have used Script# (http://scriptsharp.com/) for client-side javascript with good results. Never heard of anyone attempting node.js with it... yet.
There are some C# to JS compilers, script# and JSIL for example, yeah. But none are officially supported by Xamarin. I was just curious why they focus on all possible platforms but the web.
scriptsharp: I saw it used/had to help maintain for a rather big project...never again... :(
Used to be silverlight, moonlight? Didn't catch on.