Work to put Android onto the Nokia N900 - more commonly found running the company's own open-source OS, Maemo - has already resulted in a few experimental ROMs, but now the NITDroid project reckons they've got something almost stable enough for everyday use. Their latest release adds in voice call functionality, which most people would consider pretty important for their cellphone.
Incoming and outgoing calls are supported, as are various forms of data connection, and the whole thing is based on Android 2.2 Froyo which means it's actually potentially running a version of the OS better than the actual Android device many people are living with now. All of the Google apps are supported, and of course there's the N900's hardware keyboard to think about too. Most frustrating, though, is the apparent absence of camera support, since the N900's optics are pretty darn good. Still, you can download and have a play here.