I recently installed truphone which offers voip, works pretty similar to skype, so you can call other truphone users free if you are connected to the internet or w/3g. as with skype, if you call land lines there is a fee, and you can top up your account for such calls. so far I have only used it for international landline calls.
so there are a few options available. I would prefer if there was a skype app though, as no oneI know has truphone!
hope that helps.