I've released a new Vim plugin "arpeggio". It provides the way to define another type of key mappings called as "arpeggio key mapping" which consists of 2 or more keys and it will be mapped to a key sequence whenever all keys are simultaneously pressed. The concept of this plugin is derived from Emacs' key-chord.el.
Arpeggionoremap! jk <Esc>
" OR
call arpeggio#map('ic', '', 0, 'jk', '<Esc>')
For example, with the above definition, you can input <Esc> in Insert mode and Command-line mode by pressing j and k simultaneously, while solely pressed j and k are worked the same as the default ones.