Viber + Google Voice - free VOIP and TXT
We don't really use the phone, but it is nice to have it as a backup since we don't have a land line. It also comes in handy to call a misplaced phone, which seems to happen pretty regularly around my house.
I don't have any other devices but I would assume it would work with an iPod touch or android device as long as they have a mic and speaker. I saw some people have installed Viber on an iPad in this post.