For many years, I've used the original "Simple Green All-Purpose Cleaner" for all kinds of household and auto shop cleaning jobs, from easy to very nasty. I like the stuff because it's an environmentally-friendly, highly-versatile cleaner. It's an excellent laundry pre-soaker for greasy jeans and It's great for household cleaning, yet it's also an aqueous parts cleaner and a great garage floor cleaner. In fact, one of few places on a car you can't use All-Purpose Simple Green is on clear plastic instrument panel and gauge faces.
The most common use for Simple Green in my shop is shooting it through my Karcher G2650HH Pressure Blaster to clean suspension and underbody parts on my Corvettes and the other three cars I have in the "fleet". Simple Green is ideal for that kind of work. I've also used it "manually" to clean parts. I often use it in conjunction with Standard Abrasives Medium Brite-Rite Hand Pads in a combination cleaning and wet sanding operation prior to painting parts such as valve covers, brackets, mufflers and exhaust pipes, intake manifolds, suspension arms and all sorts of other parts.
One automotive application of All-Purpose Simple Green which demands caution is unfinished aluminum surfaces. Simple Green has been successfully and safely used on aircraft, automotive, industrial and consumer aluminum items for over 20 years, however, caution and common sense must be used. Aluminum is a soft metal which easily corrodes when exposed to water. The aqueous-base and alkalinity of Simple Green can accelerate the corrosion process, therefore, contact times with unprotected or unpainted aluminum surfaces should be kept as brief as the job will allow and never for more than 10 minutes. Additionally, I advise against using high-strength concentrations of "All-Purpose Simple Green" on aluminum parts. Do large jobs in smaller sections to achieve lower contact time. Rinse parts thoroughly after cleaning and pay special attention to flushing cracks and crevices. Unfinished, uncoated or unpainted aluminum cleaned with Simple Green products should receive some sort of protectant after cleaning to prevent oxidation.
A few years ago, to ease concern and eliminate special procedures when cleaning aluminum, Sunshine Makers, the company who makes all Simple Green products, introduced "Simple Green Aircraft and Precision Cleaner". Intended for high-tech alloys, and parts, this non-corrosive, low-foaming cleaner/degreaser contains no reagents which oxidize finishes, cause rust or degrade elastic materials such as rubber or vinyl. It does not affect the life of hoses, seals, gaskets, paint finishes and other hard surfaces. While I've not evaluated Simple Green Aircraft, I'm going to try and get a sample to try out.
Another recent Sunshine Makers product I have tested is "Simple Green Car Wash". The stuff is killer. It produces a thick, rich foaming lather which gets rid of dirt without dulling or stripping away waxed surfaces. It also rinses clean leaves minimal streaks. These days, Simple Green Car Wash is the only stuff I use to wash cars.
Lastly, about Simple Green products in-general, it's important to note that they are all biodegradable.
The Simple Green line is available at a wide variety of retail channels. I get my "All-Purpose Simple Green" at Costco. For more information, see the Simple Green web site at http://simplegreen.com/