LoudHush is a softphone application for Mac OS X. It functions as an IAX client for the Asterisk PBX. With LoudHush you can initiate and answer, hold and transfer voice calls using your computer.
LoudHush is written in Cocoa/Objective-C and integrates nicely within the OS X environment.
LoudHush is shareware software; when running unregistered, it will have exactly the same features as a registered copy, but it will always quit five minutes after being started.