Genuine Ross-tech VCDS Vag-Com cable (Hex-Can+USB).
This allows you to scan codes, check timing if you're doing a timing belt kit, or any number of other useful things. Works with all diagnostic-capable VW/Audi cars in model years 1990 through the current model year, even the latest models which require a direct CAN connection for diagnostics!
Acts as a "dongle" -- Activation for the full-function version of VAG-COM is built in! Use VAG-COM on any suitable PC, no activation codes required.
Works on Windows or Mac if running Parallels or Bootcamp (I run Parallels and it works great!).
Comes with a full 12 month guarantee against defects.