Vim

delete

:g/^#/d       # delete all the lines starting with #  g=>global d=>delete
:g/^\s*$/d    # deleting all lines that are empty or that contain whitespace only
:g!/^\s*#/d     # delete all the lines that are not comment

substitution

:%s/old/new/gc  # substitute with prompt
&   = repeat last substitution, in the current line
g&  = repeat last substitution on all lines 

repeat something

30i/I/a/A   #horizontally
3o/O        #vertically     

extras

:set paste          # allows pasting w/o indentation mess

C-x C-f         # completing file names too, as well as
C-x C-l         # completing entire lines.