Office Communicator Mobile just released to web and is now available for download from http://www.microsoft.com/downloads/details.aspx?FamilyId=BC89EC5E-5F3B-47D2-955B-B0C1DEAC94D8.
This is my absolute favorite application right now on Windows Mobile.
If you have Live Communication Server in your organisation then this is an amazing solution for corporate secure instant messaging. If you have a WiFi network as well it will allow you to utilise VOIP from a Pocket PC device too!
The English release includes:
Planning and Deployment Guide
Getting Started Guide
Frequently Asked Questions
The client is supported on both Windows Mobile 2003 Second Edition and Windows Mobile 5.0 with support for both Pocket PC and Smartphone devices! For VOIP you will need a Windows Mobile 5.0 Pocket PC however.
For more information, please visit: http://www.microsoft.com/office/communicator/mobileclient/prodinfo/default.mspx.