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OpenFire + Spark
I use this combination for one of my clients and love it. It is integrated into AD as it's supported when you setup Openfire. Openfire's documentation also mentions how to add the application as a service. It's a simple command found in the application's bin folder. This combination queries AD and I have a security group that I just add members to to populate the buddy lists automatically. It's a nice piece of software and works well.