Does the Unity browser plugin already have the C# runtime?

I'm confused - does the Unity browser plugin already have the C# runtime? or if I use C# scripting will my users have to download dotnet to play my game if they dont already have it installed?

It already has it. For webplayer builds, the end user only needs to install the unity plug in for their browser, and for Mac & Windows standalone builds, they are completely self-contained. No other runtime or frameworks are required. They really are "stand-alone"!.

Unity uses Mono for its cross-platform implementation of .NET. The Mono version currently used in Unity is fairly old though (1.2.5 I think). You can check the Mono site (mono-project.com) for details, but I've found all the core stuff I needed was available.