Updated for Anki 2.1.
This add-on enhances the Add Card screen with autocomplete functionality. Just start typing and if an existing card has a value that matches what you've typed, it will be displayed inline in the editor for you. From there you can click the value to quickly set the field.
The autocomplete search will only search the exact note type and field that you're typing into (so you won't get results from other note types or even other fields in the same note). The search also only does prefix matching, like most other autocompletes.
Thanks to acecodes and clantant for implementing support for Anki 2.1 and for maintaining a 2.1-compatible version of this add-on.