WCF itself will run on Windows only - but the services it provides can be accessed from any other language / tool.
And using WCF as a client, you can also access any other tool's services, e.g. you can access a webservice written in PHP, Java, etc.
If you want to make Platform independent web service then first you need to avoid using .net objects like Datatable, Datasets etc. And I don't think its possible to make Platform independent web service using .net without WCF.