The [2TC 15] shaders are entries in a competition that was started
by "Nimitz" at ShaderToy.com in January 2015.  It  was for shaders
that would fit into "Two Tweets", i.e. 280 characters or less. The
rule, given in Nimitz's initial post, were

   //The rules:
   // -Label your shaders with [2TC 15]
   //	-280 Chars or less (as counted by shadertoy)
   //	-Submissions end Feb 2 2015 (2 weeks)
   //	-You can use macros as much as you want
   // -Don't expect any sort of large prize

I have gathered them into a single directory for convenience.