Skype

Skype is the quite possibly the final destination in IM protocol. Utilizing a combination of VoIP (Voice over Internet Protocol) and allowing users to purchase a phone number to call ground lines, Skype is leading the way in internet communications. While you can spend money on it, the free version allows for free Skype to Skype calls. So get your friends to join in and you can soon be talking to each other for free. For a better IM client, I would still suggest using Digsby as it is still the most comprehensive IM client available.

Portable version is available at PortableApps.com