Creating Drop Down Selection Box On The Fly

I would like to create a box with the same behavior as the "To" field
in the gmail "Compose Email" page.

That is a simple text box but when you start typing it will suddently
show a list of all the e-mails (for our purpose any string would do) in my address
book that have the given substring in it.

E.g. if I type in "ga" I get all the e-mails where the "ga" substing
shows up.

Then I can select from the list and press enter on the requested
address or I can keep typing if the address is not in the list yet.