What is free VoIP software?

Free VoIP Software Phones

Ekiga
Ekiga is an H.323 compatible videoconferencing and VOIP/IP-Telephony application that allows you to make audio and video calls to remote users with H.323 hardware or software (such as Microsoft Netmeeting). It supports all modern videoconferencing features, such as registering to an ILS directory, gatekeeper support, making multi-user conference calls using an external MCU, using modern Quicknet telephony cards, and making PC-To-Phone calls.
Ekiga was previously known as GnomeMeeting.

Twinkle
Twinkle is a soft phone for VoIP communcations using the SIP protocol. You can use Twinkle for direct IP phone to IP phone communications or in a network using a SIP proxy to route your calls.
In addition to making basic voice calls, Twinkle also provides the following features:
2 call appearances (lines)
Multiple active call identities
Custom ring tones
Call Waiting
Call Hold
3-way conference calling
Mute
Call redirection on demand
Call redirection unconditional
Call redirection when busy
Call redirection no answer
Reject call redirection request
Blind call transfer
Reject call transfer request
Call reject
Repeat last call
Do not disturb
Auto answer
User defineable scripts triggered on call events
E.g. to implement selective call reject or distinctive ringing
RFC 2833 DTMF events
Inband DTMF
Out-of-band DTMF (SIP INFO)
STUN support for NAT traversal
Send NAT keep alive packets when using STUN
NAT traversal through static provisioning
Missed call indication
History of call detail records for incoming, outgoing, successful and missed calls
DNS SRV support
Automatic failover to an alternate server if a server is unavailable
Other programs can originate a SIP call via Twinkle, e.g. call from address book
System tray icon
System tray menu to quickly originate and answer calls while Twinkle stays hidden
User defineable number conversion rules