Version 2.8, originally released in July, included features like screen sharing & improved audio codecs that had been around in the Mac beta and the PC version of Skype for awhile. Version 2.8 also included Skype Access, which is a pay as you go public Wi-Fi service based on the Boingo network.
This latest update adds a bunch of fixes for Snow Leopard. Skype has very detailed release notes if you are so inclined.
Skype is a very popular solution for free audio and video chatting and instant messaging. The app also provides many paid services to allow you to call cellular and wired phones anywhere in the world at low rates, and to forward calls to your chosen phone when you are offline.
Skype [iTunes link] also runs on the iPhone, but only works on Wi-Fi. (Thanks AT&T and Apple.)
You can download the update to Skype by using the 'check for updates' feature built into Skype, or go to the Skype Mac web page and grab it there. It's a 44 MB download.
Thanks to Ben for the tip and additional information from Skype Journal