Unix pipes, cron, ssh, VNC and IRC.
Oh, and CPAN.
A Slashdotter once posted an insightful comment about cron being the 'height of computing': http://linux.slashdot.org/comments.pl?sid=323287&cid=209...
Easy to take cron for granted these days, but I have to say I agree.
As for unix pipes, I once explained how one of my shell scripts worked to a longtime Windows programmer and he literally said, "Holy shit!" when the pipes concept clicked.