We found there really wasn't much developer support for Insteon unless you wanted to buy their SDK and agree to their rather heavy-handed license agreement. Rather than go that route, we wrote our own .NET library called FluentDwelling and we open-sourced it. You can find a download link, and some get-you-started code samples if you follow that link.
The source code comes with a full suite of unit tests (requires NUnit, also free), so you can add improvements and make changes if you like.
