IPhox is a Skype alternative that has recently launched.

From the news release:

“iPhox is the revolutionary new voice over Internet protocol product which is eating into Skype’s dominant market. Available immediately for download at www.iphox.com for Macintosh and Microsoft Windows, and soon Linux, iPhox makes PC calling free, easy, fun and powerful with a wealth of leading features.

iPhox incorporates technology from Michael Robinson’s SIPphone, Inc. to ensure compatibility with the SIP open protocol standard and the ability to communicate with other networks. iPhox, Google Talk, and other SIP Client users will be able to exchange instant messages and voice calls among each other. “

Source: MarketWire