For Android, I've played around with "cSIPsimple" and "SIPdriod", both are free from the Android marketplace. To my knowledge, all Gingerbread and ICS phones have a native SIP application. In the case of my wife's Android phone, the wireless carrier we purchased it from had disabled the native SIP application, but a quick google search will provide instructions on how to apply a very simple modification to the firmware permissions which allow you to re-enable the native SIP application.
For iOS I've only tried the "media5-fone" app, which is also free from the Apple app store.
I also know that a lot of people seem to like the Bria app ($7.99) and the 3CX (free) app which are available for both Android and iOS.
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