I really like the idea of not having installed software to make this happen. Would be perfect for netbooks or Flash-capable tablets where you don't want to install Skype. Also a great alternative to Google Voice for users not in the US.

A question, though -- At €.12/minute for me in the US to call someone in London, that's nearly 6x the cost of a Skype call. Are there any chances of these rates going down? It's still deadly cheap, to be sure, but price will be a factor.