Cross-platform and instantly familiar-looking, Gizmo5--also known as the Gizmo Project--is trying to make in-roads against the domination of other VoIP and telephony programs.
Gizmo5's interface is compact and rectangular, with tabs on the top just below the Menu bar and a series of ancillary commands like Chat and Add Contact available at the bottom. There are very few differences between Gizmo's design and its biggest competitor's. A little originality could've gone a long way here.
Also similar to Skype, Gizmo provides free VoIP for computer-to-computer voice chatting as well as text chatting. It also allows for computer-to-telephone voice chatting for both mobile phones and landlines. It has one feature that Skype lacks: it lets you conference call with multiple PCs by entering in a random, shared number. Overall, the sound clarity and quality was excellent, with no lagging nor echoing. Gizmo also says it provides asymmetrical chat-client VoIP, so you can use Gizmo and your headset to talk to somebody on Windows Live or Google Talk. I couldn't get that feature to work, but it's a smart idea and hope it gets debugged soon.
Overall, Gizmo's got some good stuff and some room for improvement. If you despise Skype or are just interested in alternatives, you might want to give this one a try.