It is called sodium percarbonate. It is the active ingredient of PBW, OneStep and Oxyclean. Unfortunately Oxyclean (as in the not free) also puts bluing in with the percarbonate. What you want is the cheap stuff. It all works.
It is in fact one of my favorite cleaners. Do you have a pan that you burned something in? How about you really don't want to do the dishes? If its really badly stuck on the better. Take about 3 drops of dish detergent and a scoop of your favorite sodium percarbonate. The next day wipe it off and your done.
If you use detergent on plastic (a in brewing bucket) it will stick to the surface. Meaning you have to rinse it several times. And even then it can still screw with your brewing chemistry. Worse, if you get something (like Krausen) stuck to the top you may have the impulse to go scrub it off, DONT. It can make micro abrasions for germs to harbor in.
Really, just get some PBW, Onestep or whatever and never worry about it again.