^[ is the keyboard escape sequence for ESC, I even prefer using it on keyboards with ESC keys since it's less of a reach. It's really nice for vim and evil-mode.
I've had `jj` bound to ESC for several years now, and I know lots of people who use `jk` or `fj` too. Hitting `jj` is super quick and barely requires movement, but I've always hated the small delay after typing a `j` (while it listens for another `j` to complete the chord). I may have to retrain myself to use ^[ instead.
I'm thinking since the new Macs still have the fn key I might just remap that to Esc – I never use it anyway.