Had a customer the other day looking to do pops from Outlook. I found a couple cool products to do this.
Global IP has modified xten Pro a little bit to create a soft phone that has a Tapi driver. They call it X_TAPI Pro. Check it out here: http://www.xten.de/(3n3te155nucgdr2f4tmrcd45)/_Pages/NoFrames/PageBuilder.aspx?content=1aa9a817971e445ea0fe822aa6cebce8
It seems to be able to pop contacts from the user's contacts folder only.
A more interesting product that can work with any Tapi driver is Identapop. Check it out here: http://www.identafone.com/cidpop.html
Identapop can pull contacts no only from your local contacts folder but also from public folders. It also has the ability to log calls to Outlooks Journal for historical call information.
SIP Tapi is an open source TAPI driver that seems to allow outbound dialing only. It can dial any contact in outlook. http://sourceforge.net/projects/siptapi/
Here's a wiki entry on how to setup SIP Tapi with sipxpbx: http://sipx-wiki.calivia.com/index.php/Click-to-Dial_for_Outlook%2C_CardScan%2C_ACT%21_using_SIP_TAPI