tee
A Unix command which copies its standard input to its standard output (like cat) but also to a file given as its argument. tee is thus useful in pipelines of Unix commands (see plumbing) where it allows you to create a duplicate copy of the data stream. E.g.
egrep Unix Dictionary | tee /dev/tty | wc -l
searches for lines containing the string "Unix" in the file "Dictionary", prints them to the terminal (/dev/tty) and counts them.
Unix manual page: tee(1).