TextFree with Voice.
I have an iPhone 4 and even use this to "supplement" my AT&T minutes. I only have 450 through AT&T.
Completely free incoming calls and tons of free minutes (I currently have 2,100 minutes and counting). Although, admittedly, TextFree as of late has cut down on their install X app for Y free minutes. If you check several times a day though you can still really stockpile the minutes like I have. I've noticed they sometimes only have free minutes earning apps up for a few minutes at a time, so you have to consistently check.
Plus it's a REAL number and you can make/receive calls from/to other REAL phones. The other party doesn't have to have a Skype account, or a special app, or any other cumbersome nonsense like linked phone accounts, etc.