Digium's ISO that uses the builtin Asterisk GUI interface, very different from FreePBX and you can not go back and forth, but may be a good option for low powered embedded devices that are inadequate for a FreePBX environment.

http://www.asterisknow.org/index

Note: AsteriskNOW 1.5 is based on CentOS 5.3 and now uses FreePBX 2.5 by default.