Android obviously has a pretty active development community and there seems to be all sorts of customizations and custom ROMs out there, but from my VERY limited experience of reading various descriptions, most of these ROMs seem to behave pretty similar to stock Android? A lot seem to focus either on performance, or on upgrading devices to newer versions of Android that are not yet available for the device, etc... What I haven't found is custom ROMS that fundamentally change the nature of the underlying OS, and I'm wondering if this is just because of a lack of interest, or if there are technical reasons why this isn't possible? Is it possible to install a completely different OS on a phone that originally came with Android?