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 (currently in beta) is based on CentOS 5 and now uses FreePBX by default.