BKF (Bar Keeper's Friend) and/or PBW in mine. BKF will get it really clean for what you can touch, and PBW will get the rest (inside fittings and such).
If we're talking about my kettle then water and non scratch soft scrub pad. Boiling your wort sanitizes it and if you clean it right after you're done you don't need anything stronger than water.
It was said above, but worth repeating, Bar Keepers Friend, the powder, NOT the liquid! as that is all the store had last time I needed some. It will make a stained and beerstoned stainless kettle look and shine like new.
I always mix up a batch of Oxiclean (free variety only) with hot water from my immersion chiller to clean up all the brewing equipment at the end of a boil. I dump the bucket in the BK last and heat it up to near boiling after a good scrub with a green scrubby and run it through the valve cycling that several times to get a good clean valve and keg. Any crusties left from scorching something on the bottom of the BK, like not stirring in some extract fast enough, or any mineral deposits get a scrub when it cools down with Bar Keeper's Friend and the same green scrubby. The BKF also has the added benefit of repassivating the SS surface thereby lessening the chance of any corrosion pits forming.