Turn off Google's auto populate?

Are you talking about on Google Search?

That's called AutoComplete.

Google's statement on the matter is "it's in keeping with our vision of a unified Google search experience to make popular, useful features part of the default experience, rather than maintain different versions of Google. As Autocomplete quality has improved, we felt it was appropriate to have it always on for all of our users."

They used to have an option to disable it, but not any more.

Having said that, there are a few unofficial ways to stop it (for now, don't know how long these will continue to work).

1. Use http://www.google.com/webhp?complete=0
2. Use http://google.com/m (the simplified interface for mobile devices)
3. Use http://google.com/firefox (works in any browser)

There are other methods. None of them are ideal. But if it annoys you that much then option 1 above is probably your easiest get-around for now.