Adeptus Mechanicus Fan Codex v3
However, here's a codex that's had a lot of time and effort put into it. It's now in it's third version and continuing to be worked on. I love the style - almost a "steam punk 40k". There are also some neat rules... like being able to blow up your wrecked tanks so the technology doesn't fall into enemy hands. He's even put together a list / photos of miniatures from various manufacturers that can be used to create your army.
Click over to AdMechFanDex.com to download a copy.