Fresh from the presses, this plugin lets you create Indigo devices for certain Denon AV receivers. If you have a Denon "AVR" receiver with a serial port or Ethernet port on its back, this may be useful to you. If you have a recent Marantz receiver like that, it may also work for you (Denon bought Marantz a few years ago, and the computer interfaces are very similar).
What can you do? A lot, actually. You can set the volume, switch sources, fiddle with surround modes, invoke presets, set the tuner, and so on. You'll see the major operating characteristics of your receiver reflected as device state in Indigo, and so you can trigger events when things change (the receiver switches to Blu-Ray, the volume goes above -20dB, zone 2 turns on, and such). Read the help.
What's the catch? You need a network or serial interface, so the really low-end Denons are off limits. Denon receivers don't announce their capabilities to the computer, so the more different yours is from mine (an AVR3312CI), the more work you'll probably have to do to make this work well for you. And of course, this is currently tested on a sample of one (mine), so you'll be helping improve things with your feedback.
More information and download from here: http://www.cynic.org/indigo/plugins/online/denon-av.html.