Just another vote for N900.
I have a bunch of devices here, including an Openmoko. There are some
Android (and some Windows CE phones thanks to the HaRET bootloader) that
have been jailbroken or hacked to run other software on them (typically
Ubuntu) or to avoid the limitations of their original firmware. Those
modifications are usually quite experimetal, expect many glitches,
missing and not working drivers, and a lot of patience and time to make
them barely usable. It is OK if you just want to develop or hack them
for fun, but if you actually expect to use the phone then go for N900,
there is no other comparable alternative right now.
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