If you define killer apps like "better than anything, that does the same job, on other systems, there are none that I'm aware of.
But thats the same for AROS or MorphOS.
The Software that is available is good but none of the available stuff would kill any other software or system in terms of stealing their market.
The closest thing that comes to my mind thinking about killer apps would be things like a VideoToasterHD and the latest LightWave or stuff like Cinema4D.Or just an Office that is compatible to MS-office(maybe that exists). Of course Steam would be nice. I'm not a big fan of the above mentioned SDL games. ;-)
I think even a driver for modern 3D HW would turn out to be some kind of killer app.
But without an industry behind us, I don't see that happen in 2015.