The integrated VoIP support in Windows Mobile 6 is pretty nice. You can set it to automatically use VoIP "Internet Calling" when it's available through a WiFi connection (or 3G with a reg hack). It supports most SIP VoIP services. I use Voipbuster. Many devices don't come with the VoIP component though.
It sometimes has issues automatically and reliably connecting to the VoIP service, so I'll just manually turn it on after connected to WiFi. Provisioning is a bit difficult if you go the CAB file creation route, which is required on "Standard" devices, but Shap has created a "Setup VoIP" utility to make this easier on WM Professional. You might also have to create your own ipdialplan.xml file. This xml file defines how to dial phone numbers in your addressbook when using the Internet Calling account.
After you get everything working, the VoIP is fully integrated with the Windows Mobile dialer and you can make VoIP calls using any of the normal calling methods including Microsoft Voice Command via a Bluetooth headset.
That's one of the reason's I've dropped Skype!
The voice quality on VoipBuster is better than regular GSM, however you might notice a slight delay between the voice transmission. Other services might have better results. I would imagine it would work better if you use your own internal SIP VoIP server.