http://fredricksensoftware.us - This site has a few nice Insteon homebrew applications. Group Commander lets you create groups in your devices. Groups are how Insteon allows one button to control mulitple devices. Device Controller is an application that allows you to turn devices on and off from your computer. Copies of SDM are availble here for download as well.

http://www.cwfam.com - Ken Mill has some nice utilities here. One allows you to set timers in your PLC much like Smarthomes free Timer Software. Many people prefer Ken's InHomeFree interface to the TimerSoftware though. He also has a nice Command Line Interface to the Insteon network.

http://power-home.com - Dave created PowerHome a while ago for X10 and IR interfaces, but he has recently added Insteon support. It is excellent software, but not free. The demo is free, and you can see that it is quite powerfull. The forum is active, and Dave is great at fixing bugs and implementing new features.

http://www.accessha.com - This is one of the most popular forums for talking about Insteon hardware. There are some very smart people here that can answer your tough questions (like BLH and KenM from cwfam.com).