A simple demo showing how to use the jQuery UI autocomplete widget in conjunction with Google Maps geocoding service.
See it in action here: rjshade.github.com/gmaps-autocomplete/
Some more explanation here: rjshade.com/2012/03/30/Google-Maps-autocomplete-with-jQuery-UI
I've had success using this with the SimpleForm gem:
= simple_form_for(@post) do |f|
= f.input :address, :input_html =>{:id => 'gmaps-input-address'}, :placeholder => 'Start typing a place name...'