I know these API's are used for doing something easier than WCF (in terms of config and performance) for .NET, but I wanted to know if these API's can be used on other frameworks too?
All web service frameworks just serve JSON/XML/SOAP over HTTP - this is the path of greatest interoperability.
As for all ServiceStack demos, they're all consumed live with HTML/JavaScript, so no .NET on the client to speak of. Of course if you use ServiceStack's C# clients you get the benefit of a strong-typed sync and async clients.