Skype has offered an iPhone client for quite a while, but with one big restriction: you could only make calls over a Wi-Fi connection. That changes with the latest version, which lets you make calls to other Skype users, or using your Skype credit, over a 3G connection.
Obviously, you wouldn't want to do that if your iPhone plan doesn't have a healthy offering of data. It's also worth noting that while the software will initially let you make Skype-to-Skype calls for free over 3G, that won't be the case long term; at the end of 2010, Skype will announce rates for those calls, alongside the existing rates for calling conventional phones. (Calling Skype-to-Skype over Wi-Fi won't be charged, according to the company.)
The new version runs on all iPhone models, and iPod Touch models from the second generation onwards. You can also run it on an iPad, though not to make 3G calls — it's largely a manage your contacts and send instant messages option on that platform. Outside the Apple world, Skype runs on various Nokia phones, as well as coming pre-installed on a number of phones.