I recently started using a cleaner called LA's Totally Awesome. Its non-toxic and biodegradable and unlike the above mentioned cleaners, non-flammable. I get it at the Lowe's I work at and have been using it to clean some old, grimy wrenches, my PitBoss smoker and some metal toys I piced up at a garage sale/ (I hope it's not another simple green )
Also I have been looking at a sonic cleaner. They have really dropped in price. I have seen great results cleaning old Harley engine parts using one of these cleaners and water.