I inherited a few small ork armies over the year and decided to start painting everything when Assault on Black Reach came out and I got those orks.
And here's a quick run through of all the existing units.
Here are some detail shots of the main ork units.
Checkboyz (they all have black and white checks on their shoulders to identify them)
Supporting these are:
Da Big Gunz
Deffkopta with buzzsaw
Kommandos (copied from someone else on teh intrawebs)
Shokk Attack Gun
Mad Dok Grotsnik