linphone

www.linphone.org

ekiga

ekiga.org
linux (gnome)

Does not support RTP encryption.

You can get a SIP address at ekiga.net

qutecom

QuteCom is the new name of the WengoPhone project
www.qutecom.org
linux (KDE), osx, windows

Pretty and slick, but does not allow multiple accounts.

twinkle

www.twinklephone.com
linux (KDE)

Seems to be the best performing and have the most features, but confusing as hell.

jitsi

jitsi.org
Windows, os X, Linux (All Java supported)

Seems the most portable and easy to install.

blink

icanblink.com

You can get a SIP address at sip2sip.info

x-lite (free for download, eula)

www.counterpath.com/x-lite.html

Runs on linux, osx and windows.

SJphone 1.65 (commercial)

www.sjlabs.com/sjp.html

Runs on Linux, M$, Mac,
But i’m try only in M$

SFLphone

sflphone.org

Runs on Linux only (GNOME,KDE,cli)
provides Signalling and voice encryption (TLS, SRTP)