ODroid C2 and XU4 have been making progress, though last I saw they weren't really stable yet. I think HardKernel, the manufacturer, is providing some level of support for the mainlining effort.
Next Thing says the C.H.I.P. (or at least the C.H.I.P. Pro) runs mainline Linux, but that's not really comparable to the current Raspberry Pi.
Does it have support for the graphics though? That's the first big stumbling block these alternate SBCs run into. Without support for graphics acceleration you won't be able to play video properly or run fancy desktop apps. It's a huge drawback these days.
Next Thing says the C.H.I.P. (or at least the C.H.I.P. Pro) runs mainline Linux, but that's not really comparable to the current Raspberry Pi.