I like this but i can't decide if it's technically abuse or not. The paste command will happily parse - (meaning read from stdin) multiple times, so for transposing a list into a table:

  file.txt:
  line1
  line2
  line3
  line4
  line5
  line6

  $ paste - - - < file.txt

  line1 line2 line3
  line4 line5 line6

Combine with the column command for pretty printing. I seem to find a use for this pretty frequently.

I like the simplicity of this one but it's not very useful day to day:

  $ echo *

As a replacement for "ls".