SIPphone: www.sipphone.com
Skype: www.skype.com
SIP Broker: www.sipbroker.com
Google Talk: www.google.com/talk
Vonage: www.vonage.com
Lingo: www.lingo.com
Total Talk: www.totaltalk.com
Asterisk: www.asterisk.org
SIPfoundry: www.sipfoundry.org
GnomeMeeting.org: www.gnomemeeting.org
Damien Sandras' GnomeMeeting Blog: blog.gnomemeeting.net
KPhone: www.wirlab.net/kphone
CounterPath: www.xten.net