the calling procedure looks to be a bit of a pain, but it is free. of course, you'll still be charged for a local call, so it's useless unless you're making an international call to a friend in one of the listed countries. the list of countries it can be used in is still pretty limited. http://www.talkster.com/Countries.aspx
i can't think of any reason that talkster couldn't be used on any computer or internet tablet. from what i can tell, skype or some other voip service (or a regular phone) would need to be used to make the call after talkster sends you and your friend dedicated phone numbers. am i wrong?