Power Plan Switcher is a simple Windows tray app which only task is to switch to "Maximal performance" plan on AC connect, and to "Power saver" on disconnect.
It also shows current power plan, allows manual switching between the default schemes and displays power cable state and battery percentage.
And it is as minimalistic as possible.
Well, default Windows behavior is to use two set of settings for each power scheme: AC on and AC off. As for me, it's easier to switch between those schemes.
Written in C# (3.5, I guess). Well, I'd be more comfortable using something JVM-based, but, honestly, Java on Windows desktop still sucks.
The full version is here. It also acts as an installer, copying the executable to the startup folder on first run.