Matt Bettcher has ported an open source emulator for the famous NES platform to Windows Phone 7. He says the code, at present unoptimised, runs at 10-20 frames per second on this Samsung Focus, which is not too bad at all.
He has however spoken to Microsoft, who has advised him the project will not be allowed in Marketplace, and he is asking for some community support to try and change Microsoftâ€™s mind. Of note is that even Apple has finally allowed emulators into the App Store.
Matt intends to release the app for free on Marketplace, and also make the app, which itself is based on Open Source code, open source. If you wish to join Mattâ€™s campaign comment on his YouTube channel here.