Windows: If you find yourself frequently toggling between display resolutions, HotKey resolution changer makes it as simple as hitting a hotkey combination.
If you frequently change your screen resolution, HotKey Resolution Change (HRC) makes short work of the task. You can assign up to nine shortcuts for various resolutions and swap between them with keyboard shortcuts.
HRC would be a great addition to a media centre to enable easy switching between high resolutions (for HD movie watching) and lower resolutions (for comfortable general computer use and web browsing). Had I known about HRC when I set up the Windows box in my living room I could have forgone spending time writing a series of batch scripts to achieve the same effect.
HotKey Resolution Changer is open-source, Windows only. Have a tip or trick for easy monitor wrangling? Let's hear about it in the comments.