intersect of files

I have to share this page!
http://www.pixelbeat.org/cmdline.html

I spent half an hour finding some shell trick to get the intersection of several files, but in vain. Then came cross the above page. It has various useful commands (and in several languages). The section of 'set operations' is exactly what I want. How come I didn't know 'join' and 'uniq'!