I've written a sketch to control various features of my saltwater aquarium. Right now it only handles the lights, but someday... 
The hardware and the sketch are introduced on my blog at http://imperfect.olbert.com/2010/07/11/aqcontroller/.
I've also written a Windows .NET Framework app to make configuring the script easier. It's described at http://imperfect.olbert.com/2010/07/14/aquarium-controller-windows-interface/.
Enjoy!
- Mark