Citrix XenDesktop offers Windows Server 2008 R2, Windows Server 2012 R2, and Windows XP, 7, 8 client desktops. It runs on the HDX protocol, and service can be obtained through authorized service providers. Prices vary depending on the provider.
One may wonder why Microsoft is not in this discussion. Microsoft has its Azure RemoteApp platform, however, it is not a complete DaaS solution as it only serves applications from the cloud, not desktops. Some analysts suggest this may be due to complex licensing issues that still need to be resolved. In any case, the company appears to be waiting on the sidelines at this time.