Is there an iMessage client for Windows?
As of May 2016 the only official iMessage clients are for iOS and OS X. Unfortunately you're out of luck using it on Windows.

It's very unlikely that any iMessage clients for non-Apple platforms will be released in the future. It's a major selling point for Apple's platforms, and therefore not in Apple's interest to make a Windows (or Android, Linux, etc.) version. Additionally, it has some fairly complex encryption and security protocols, which likely make it very difficult if not impossible for an unofficial client to be developed.