I have had good luck using X-Lite with Asterisk. I have multiple users who VPN in and then use X-Lite to hook up to the PBX to make calls. I use the free version and it is for both Linux and Windows. 8)
EDIT: I have only limited experience using the video portion. I think it worked well when I did use it though. Not sure what codecs I was using as it was awhile ago.
Actually:
I sampled many, I mean many… most of them do the job with ackward interfaces sometimes.
All those 3 packages (the add on has some work to be done for the interface…) but they are easy to operate…
if the user does NOT need to transfet then Free Xlite is good.
If the user needs to Transfert calls we use Bria professionnal $$
If the person is an heavy Outlook user then we use Bria addins for outlook which does not require the 2 other to work… it is a stand alone product. I still does not know how to conference people using the add on though:-). I showed this to some users and they want it… (we are just starting a slow voip integration, asterisk is between old pbx and pots)