What version of Windows server is needed for RemoteApp functionality?

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http://serverfault.com – We have a Windows desktop app that we want to make available over the Internet to a few users. What version of Windows Server do I need for that RemoteApp functionality? Do I need CALs too for this, or...? I tried to figure this out via Microsoft's website, but I couldn't get a clear bead on how exactly the licensing works, or what versions of Server support the RemoteApp functionality. (HowTos)