The Skype mobile software is free, you can download it to your phone directly. Using this anyone can receive calls, IMs and see who is online in your skype contacts but if you want to call it doesn’t support all the countries. Currently it only supports Brazil, Denmark, Estonia, Finland, Poland, Sweden and UK.
It is definitely a good option to call your contacts while on move but you need to consider the charges associated with it. Check the rates calling, receiving, IM etc here.