After making my libarian I had a spare model left from the legion praetor set and figured he had enough artifice to warrant making him a techmarine. Currently I think he'll be my master of the forge but that might change if I get more tech marines. I'm going to use him to man a thunderfire cannon. I think I'll use one of these as a thunderfire cannon.
I built the backpack from a missile launcher arm and sergeant chronus' power pack/arm.
The shoulder pad is from the dark angel ravenwing sprue. I carved up the skull to remove the hood. I used this pad because I wanted the raised metal guard; it makes the model look a bit broader.
Since the sword that comes with this model didn't really fit in with the techmarine look, I added an auspex. I figure he should have one for tinkering with stuff :).