OpenMoko
Hackable at any level

System-level hacking (bootloader, OS)
entire bootloader from very first instruction FOSS
entire kernel including all drivers FOSS
JTAG accessible on debug connector
serial console on debug connector
debug board (USB JTAG adaptor and USB serial converter)
un-brickable through emergency boot loader in read-only NOR flash (GTA02)
DFU (Device Firmware Upgrade) for full-system re-flash via USB