Some guy (a real standup comic by the way) came though the neighborhood last weekend selling a product called, ADVANAGE.
I was skeptical of his claims but it removed rust on the garage floor, cleans grease, shower doors and glass, windows, etc.
The usual concentration is 20 parts water to 1 part water concentrate, but can be higher for tough cleaning, like grease and oil stains on the garage floor.
Needless to say, I bit and bought a kit that has a scouring pad, spray bottle, and a quart of the concentrate.