I'd like this thread to become a trove of information for anyone interested in building an Arcade Machine with an RPi, and AGK.
To get it started, here's a neat trick to save GPIO connections:
If you need lots of buttons for your DIY arcade machine (or whatever project you're working on) you can save all your GPIO connections by taking an old USB keyboard controller circuit and hook the connection from that to your buttons instead! This has the added benefit of giving you all the buffered keyboard input goodness, along with ready-made commands such as GetRawKeyPressed, GetRawKeyReleased, GetRawKeyState, GetRawLastKey etc!
Caution: This method will suffer from the "N-Key Rollover" effect where certain combinations of keys (or Arcade Machine Buttons!

) will glitch (known as jamming (keypresses aren't registered) or ghosting (erroneous keypresses registered)). Care should be taken when arranging your buttons to prevent multi-press combinations that could cause jamming or ghosting.
Using a keyboard input circuit for your buttons leaves all your GPIO connections free for bells and whistles!
I'll have a crack at this over the weekend and post up some pics of my efforts.
ArcadeWorldUK looks like a very good resource for Arcade Machine kit (thanks Steve for nudging me in their direction!).
Inexpensive Arcade Joysticks & Buttons kit:
http://www.arcadeworlduk.com/products/2-Player-Arcade-Joysticks-And-Buttons-Kit-No10.html
Flat-pack cabinets:
http://www.arcadeworlduk.com/categories/Arcade-Machines/Flat-Pack-Kits/