Some mobile phones are compatible with Skype. Skype can be used on an iPhones, Windows Mobile PDA, some Nokia phones. My Windows Mobile phone allows to make calls using Wi-Fi or 3G.

As for Skype Wi-Fi phones, you can use IPEVO S0-20 WiFi Phone for Skype, for example.

p.s. Note that there are many other providers that allow to make free calls over Internet. They allow you to make calls to landlines and mobile phones as well. Many SIP providers are cheaper than Skype. And you can find software for your cocomputer or mobile phone, as well as hardware for making SIP calls.