One of their better known pieces is this guy -
I have seen a few people use this as the base of a conversion for Orks as well as Space Marines with good results.
Tobsen77 recently released this guy and it just screams Mega-Dread, or maybe even a smallish Stompa -
The model looks pretty plain, but with a bit of guitar string and Ork Glyphs this thing would just scream Ork.
There are tons of other great models on this site - some more suited to Warhammer 40k than others - and it is a great source of inspiration.