Atom by GitHub:
React by Facebook:
[updated] Default icon for apps on Facebook:
[update Wednesday 12 March 2014]
Rocket Yoga by Michal Stefanow:
(actual design work by Mateusz Sarnecki)
I think that apple was first, I remember to use them as reference when crafting the brief:
If we go back to 2011:
I believe I was one of the first to use Cabin Sketch, soon after I started seeing it everywhere! See GoogleTrends graphs below: