[sph]

There are three languages worth learning that expand your mind: Lisp, Forth and Tcl. Despite all exhibiting homoiconicity, they couldn't be more different from one another.

(I'd include Rebol but it's as mind-blowing as it's dead technology from a lost timeline)

[oldes]

Rebol is not dead. It's just invisible to the mainstream. There was a new release (3.22) last week.