Most devices or at least a few PDA phones come with Windows Mobile 5.0 which is a neat OS for most phones. Microsoft Announced Windows Mobile 6.0 a while back and it will be releasing updates for most Windows enabled phones. However it is not yet released from AT&T or Palm, the Rom has been released and is available for download at a few select sites.

The following WM 6.0 includes the following:

AT&T Mail, AT&T Music, Games, MS Office Mobile, Active Sync, Calculator,
Camera, Cellular Video, File Explorer, Get Telenav, GetGood, Get MobiTV, Internet Sharing, Notes, PDF Viewer (Picsel), Proxy Manager (ATA&T Proxy),
Quick Tour, Remote Desktop Mobile, Search, SIM Manager, Sounds, Tasks, Windows Media, and Voice Command 1.6 is included.

This will is only said to work on the Palm Treo 750 from Cingular, now word yet on other releases.

Source