If you have a function that you use a lot, you may find it interesting to use a custom command for it. Try this:
:command Foo echo('Hola!')
Now invoke the command with ':Foo' and Vim should say hello. Neat, huh? This is especially useful (and dangerous) when combined with cabbrev, like this:
:command! Foobar echo('Nope!')
:cabbrev close Foobar
If you try to :close a document, Vim will now say "Nope!". Other than using this to mess with someone's Vim session, you can replace builtin commands with your own tweaked functions. I tend to use that quite frequently in my own .vimrc.