Matt Robert, VK2TVK writes in the APCO25 ham radio list:
I would like to let you guys know about an open source P25 decoder/analyzer project that has been running for a little while now. OP25 started as the thesis for a Computer Security PHD student that is researching the security aspects of the APCO25 standard. The software is being expanded into a decoding suite that can gives various bits of information about a received signal and there is also provision for a software based repeater.
OP25 is based on the open source GNUradio package, and it is compatible with either the USRP software radio peripheral, or a conventional NFM radio connected to a sound card via a discriminator tap. One contributor is also working on adding transmit code with the intention of giving a software based P25 repeater that can be run off conventional PC hardware. Currently all the major contributors are HAM operators.
At the moment OP25 is considered pre-alpha (i.e. developmental) and we are looking for people that are interested in becoming developers/contributors to the project. The project is designed to run under GNU/Linux and it requires a bit of skill to get it going. It's definitely a HAM radio project at this stage and that's why I figured it would be appropriate for this list.
Shown here are are some sample screenshots for the curious.
Please feel free to have a look at the project homepage at http://op25.osmocom.org and there is also a discussion list at http://tech.groups.yahoo.com/group/op25-dev/
[Update Dec 2009]
Here is an interesting amateur video, that demonstrates the OP25 code in action with GNU radio using the USRP.
[Update April 2012]
Here is an updated video, showing OP25 code with GNU radio using a cheap USB TV tuner as a replacement to the USRP.