Projects using APRSdroid

Bluetooth SPP based KISS TNCs

Kenwood TH-D7 2.0 with Bluetooth module (Video, Gallery)
ICom D-Star gateway using ATMEGA and BTM-222 bluetooth module (German)
TinyTrak 4 with Bluetooth daughterboard
Battery-powered Bluetooth KISS TNC created by Rob wx9o / Mobilinkd, buy now

Sound Card Interfaces (AFSK)

Attention: while the sound card interface is the most inexpensive way to connect APRSdroid to a real radio, it has several drawbacks compared to Bluetooth TNCs:

Higher battery usage (the CPU must be running all the time to decode the audio stream; this is still less than GPS though)
Lacking shielding: the audio cable is a direct electrical connection between your phone and your multi-watt RF emitter. This could kill your Android!
Accidental transmission of Android sounds - every sound that your Android plays (low battery, chat notification, etc) might be transmitted by your radio on the busy APRS frequency!

That being said, here are the sound card interface projects:

Very detailed Howto for Wouxun/Baofeng, with APRSdroid configuration
Sound cable to UV-3R (Czech)
English forum writeup of UV-3R setup (no schematics)
Cable for connecting APRSdroid to a Baofeng UV-82 Radio (APRS via RF) minimal cable with two resistors

USB Interfaces

Attention: The USB cable is a direct electrical connection between your phone and your multi-watt RF emitter. This could kill your Android!