Inspired by Vidar Hokstad’s two-iframes gadget emulator, I have created one that is actually backed up by a real database. It polls every 6 seconds for updates, so it’s not as immediate as Google’s (yet) but that would actually make it easier to discover concurrency problems with gadgets.
Each generated URL points to a specific gadget with a specific instance. You can share these URL’s on multiple machines (or windows if you are testing alone) and they will synchronize data.
http://wave.thewe.net/emulator
Code on GitHub: http://github.com/avital/google-wave-gadget-emulator/tree/master