The Work In Progress forum http://z11.invisionfree.com/Work_In_Progress/index.php is one of the best-kept 40k secrets on the web, an elite group of mind-blowingly cool convert-a-holics and scratch-builders.
Their Inquisition board includes plenty awesome of Adeptus Mechanicus conversions.
Some of my favourite Adeptus Mechanicus related conversions on WIP are those made by Malton Necromancer, also shown on his photobucket.
His tall skinny Grand Artificer was a clear influence on my Techno-Magi, his heavy praetorian/ogryns made from WFB Ogres are excellent, and I love all his walkers and skitarii and general craziness.
But my favourite of all the WIP Adeptus Mechanicus posts has to be this heavy dreadnought made GreyDeath (the chap who runs the forum). Mixing sentinel legs and a dreadnought torso looks so good! I really want to make some of these for my AdMech at some point- possibly I'll use this as the basis for a Knight Titan?