Vi and its variant vim are essentially an improvent above the old "ed" line editor in unix. Vi has support for regular expressions, mutliple buffers and more yet it is extemely lightweight.
_Where can I get vi?_
Vi is included in most operating systems. If you're using an inferior operating system and wish to emulate a real operating system you can download:
gvim GVIM (graphical vi improved!)
cygwin Cygwin (bash shell and ports of many UNIX tools for Winblows)