Android phones with native SIP client?
SIP comes directly with the Android system, as you said, starting with Gingerbread. You find it in the system settings: Call Settings, at the very end: Internet Call Settings:



For more details, you can also check with the page Android SIP Client Configuration and Review, which explains you all the details.

As it comes with the Android System itself, all devices with Gingerbread or higher should have it.