XMPP

net/skype21
comm/asterisk
net/teamspeak (client and server)
pidgin
Coccinella
Gizmo
Jabbin


Non-XMPP

wip/sipwitch
wip/py27-shtoom
wip/twinkle
CounterPath X-Lite
net/Ekiga
Kphone
wip/linphone
WengoPhone
SpeakFreely
Sjphone
SIPSet


Of the mentioned VoIP packages with XMPP support, Jabbin is what I think best.
It gave the best hearing quality and worked on a range of different neworks and speeds without ruining the sound quality.
Its ability to send and receive files at such incredible speeds was also the best, with its only competitor being skype in this scenario, and it was much faster than skype.

Worked well on Ubuntu 12.1 however lagged a little on Windows 7