Express Talk VoIP Softphone is a freeware application for Mac OSX and Windows based computers that allows you to make free PC to PC or PC to Landline/mobile phones through any VoIP gateway. The software is also available for Windows Mobile based Pocket PC devices. Express Talk also works as an extension with free VoIP Virtual PBx Systems.

Here are the main features of the software:

-Make internet phone calls free direct PC to PC, or PC to phone via a VoIP SIP gateway provider
-Supports up to 6 lines on one phone with the ability to put calls on hold.
-Works with a headset or in speakerphone mode with just a standard microphone
-Includes data compression (GSM, uLaw, ALaw, PCM and G726), echo cancellation, noise reduction, comfort noise and more.
-Can be configured to work behind NATs and Firewalls
-Supports caller ID display and logging.
-Includes a phone book with quick dial.
-Integrates with Microsoft Address Book.
-Plays on-hold music to callers on hold

Some other features such as call transfer, recording phone calls, conferencing and Do Not Disturb are available in the Business Edition version of Express Talk.

Download links for the freeware versions-

Mac OSX version here
Windows version here
Windows Mobile Pocket PC version here