# To tee stdout to a file:
ls | tee outfile.txt

# To tee stdout and append to a file:
ls | tee -a outfile.txt

# tee would take the output of the first command in the pipeline and copy it 
# to standard output (the terminal) while also making copies of it in the files one, two and three:
somecommand | tee one two three

# write 3 to /proc/sys/vm/drop_caches (with root rights)
echo 3 | sudo tee /proc/sys/vm/drop_caches

# in Vim:
# The structure :w !cmd means "write the current buffer piped through command".
:w !sudo tee %