Acetone is a good solvent for cleaning almost anything polymer based (as stated, it has little immediate effect on cured resin, but I would not soak a resin part in it.) It is the active ingredient in nail polish remover It is very aggressive to styrene. It has a very high vapor pressure and is extremely flammable.
Acetone is considered to be of low toxicity. However, it is extremely dangerous to the eyes. Always wear adequate eye protection when using. High vapor concentrations may cause drowsiness, irritation, nausea, vomiting, and if prolonged can result in serious injury or death. It dries the skin and mucous membranes, and may cause or aggravate dermatitis.
Acetone attacks lipids, fats, and oils, and for that reason is not always a good solvent for enamels and oil based paints, sometimes making them gummy, sticky, and even more difficult to remove. Test first!
Acetone is moderately miscible with water. However, it is sudden and complete death to acrylic and other aqueous paint systems.
As others have stated, it can be used as a general purpose cleanter and thinner in various applications. It is the best debonder for cyanoacrylate glues.
Because of its high vapor pressure, it is not a good solvent cement.