HTH pool shock is the shit, there is thread about it here on RIU with some kick ass information.
I've also heard nothing but good reviews about Pythoff, people have been using it with great success (for preventing and treating), I've ordered it and will start using it soon. The problem is, is that it's basically chloramine and water and the price is a bit high (but they can be bought seperately at a very low cost) but sod it. If it works then I'll be more than happy.
Is this for a "clean up" or something you want to add to your nutrient solution?
Pool shock..calcium hypochlorite...is the most economical by far. It also adds calcium. If you have hard water, bleach...sodium hypochlorite...would be the way to go.
Both are chlorine, pool shock is just much more concentrated.
H202 is also kick ass, people cry about ti sometimes, but I'm betting they never even tried it out. it's not poisonous to your plants and does well. Some even cured root rot with it, but that is rare.