Key features include:

- Support for multiple SIP accounts
- Detection of IP address changes
- SIP account sign-up at http://sip2sip.info
- Bonjour discovery mechanism
- Integration with MacOSX Address Book (MacOSX)
- Integration with Google contacts (Windows and Linux)
- Answering machine
- Displays RTP end-point information
- Automatically play and resume iTunes
- Multiple parallel calls
- Silent mode (do not ring on incoming call)
- Mute microphone
- Audio recording

And more. See the full feature list here.

Empathy uses Telepathy for protocol support and has a user interface based on Gossip. Empathy is the default chat client in current versions of GNOME, making it easier for other GNOME applications to integrate collaboration functionality using Telepathy.

Key features include:

- Multi-protocol: Google Talk (Jabber/XMPP), MSN, IRC, Salut, AIM, Facebook, Yahoo!, Gadu Gadu, Groupwise, ICQ and QQ. Supports all protocols supported by Pidgin.

- File transfer for XMPP, and local networks.

- Voice and video call using SIP, XMPP and Google Talk.
Some IRC support.

- Sharing and viewing location information.

- Private and group chat (with smileys and spell checking).

- Conversation logging.

- Automatic away and extended away presence.

- Automatic reconnection using Network Manager.

It is distributed as free software under the terms of the GNU General Public License. It was the default VoIP client in Ubuntu until October 2009, when it was replaced by Empathy.

Ekiga supports both the SIP and H.323 (based on OPAL) protocols and is fully interoperable with any other SIP compliant application and with Microsoft NetMeeting. It supports many high-quality audio and video codecs.

Key features include:

Call forwarding on busy, no answer, always; call transfer; call hold; DTMF support; basic instant messaging; text chat; register with several registrars and gatekeepers simultaneously; ability to use an outbound proxy or a gateway; message waiting indications; audio and video.

Dimdim Open Source Community Edition has most of the features of Dimdim Enterprise and is based on open source streaming and media components.

Designed for ease of use, WebHuddle overcomes many of the common challenges faced by other virtual meeting applications on the market — such as high-price to entry, compromised network security, large client downloads, and unreliability — resulting in a frustrated IT department and less than satisfactory business results.

Core features include:

- Cross-Platform: Use any Java-enabled computer (Linux, Windows, Unix, Mac) Learn more

- Simple: No installation needed — client runs in web browser Learn more

- Small: Thin client (about 125 KB) — loads quickly Learn more

- Secure: All data encrypted by HTTPS protocol Learn more

- Open Source: Leverage the many benefits of open source software, including value, transparency, and flexibility. If you find WebHuddle useful you may download it and freely install it on your own network.

It offers the following features:

AppleScript: A powerful dictionary for easy scripting.

IRC, SILC & ICB Compatibility: the result of years of chat experience.

Plug-In Love: Unlimited extensability through Plug-Ins.

Styled For You: Pick the look for your conversations.

Colors of the World: Fully compatible with mIRC colors.

Unintrusive Interface: Sleek multi- or single-window modes.

File Transfer Medly: Full DCC transfer support with resume.

They’re Your Buddies: A buddy list for your IRC friends.

Express Yourself: Use the emotions that speak for you.

Multiplicity: The freedom to chat on multiple servers.

Its main features including the following:

Display picture transfers; fast file transfers; custom emoticons and nudges; winks and offline-im messages receiving; rich and colorful chat styles and logs; live Mail e-mail notifications and more.

Asterisk offers both classical PBX functionality and advanced features, and interoperates with traditional standards-based telephony systems and Voice over IP systems. Asterisk offers the advanced features that are often associated with large, high end (and high cost) proprietary PBXs.

Amongst its main features are the following: Sign in with multiple accounts; Fast file transfering; Simultaneous sending & receiving webcam; Offline messaging; Extensive event notifications; User defined event actions; Single window (tabbed) conversations; Customizable contact list; Customizable message views; Custom status icons; Resource saving (Webcam streams, Display pictures, Emoticons) and more…

Some of aMSN’s features include: Personal messages, “What I’m listening to” personal messages (from a third-party plugin; includes support for music programs like Amarok in Linux); Contact list and display name retrieval from Microsoft servers, Offline messaging; Logging in under any status; File transfers, sending and receiving; Replication of Windows Live Messenger’s look and feel and much more…