Enough of these started accumulating to warrant a node of their own:
Fully Open Clients:
(i.e., that you can configure to use with any provider)
Multi-Platform:
- X-Lite, a SIP softphone for Windows and Mac OS X
- eyeBeam, a SIP and H.323 videophone.
- SJPhone, another softphone (nagware)
- SflPhone - Linux-only for now, but stated as portable.
Windows
- Express Talk - Windows, freeware.
- sipXezPhone - lame name, ugly UI, but not half bad.
- Dialexia - unfortunate name, rather interesting Windows client. Time-based license.
Linux
- Twinkle - Qt-based.
- GnomeMeeting - evolving to SIP from H.323.
- minisip - Linux/PDA client.
- Cornfed - Linux-only. Looks simple and easy to use, but is closed-source.
- MythPhone - for MythTV.
Closed (Preconfigured) Clients:
Mac:
- MegaFon - not free, apparently pre-configured for its own service.
Multi-Platform:
- OpenWengo
- Ineen - rebranded eyeBeam with hard-coded settings, has a Mac OS X version.
- PhoneGaim - SIP (very badly) integrated into Gaim (the Windows beta completely trashed my IM setup)
- Gizmo - can't get it to work at all behind a firewall.
Windows:
- Pulver Communicator - Windows, integrated with IM and FWD. Also includes a (rather crummy) RSS aggregator. Can be unlocked for other SIP providers, but that doesn't rate it as an open client.
- Sapo - mixed Jabber/SIP IM client with voice and video support.