Rotten flesh is probably the easiest to obtain of all the mob drops. However, what are its uses? There are not many, but here are the best uses of zombie flesh.

#1. Food Source
Sure, it doesn't give you much hunger points (two bars), and those points are quickly taken away from the 80% chance of the hunger effect. However, if your in danger of starving to death or need to regenerate health, if push comes to shove, eat the mystery meat. If your game is on Peaceful, you won't even be affected by hunger.

#2. Taming
Wolves are pretty picky when it comes to taming them. They never ate their veggies as cubs, and they certainly won't now. However, they will accept rotten flesh, and in exchange they will love you. It can also be used to regenerate their health (though cooked meats will be more effective) and allow them to breed.

#3. Emerald Farming
What? Emerald farming? How could meat stripped off from a contaminated poor soul possibly be used for emeralds? I don't know either, ask the Villager Priest why that is, because he will give up his life savings for some of that rotting produce. Maybe he's/she's a cannable? Whatever, idk. To find a priest, you must either:
a. Cure a zombie villager with a weakness potion and golden apple (and get lucky).
b. Find a village with a church in it (you know, those large cobblestone towers?).

#4. Crafting
Now hold on right there, Quark. Rotten flesh isn't in any crafting recipes! You may be right, however that only applies for vanilla minecraft. Many mods use rotten flesh in many materials. In Hexxit, you can cook rotten flesh to get leather. In Crash Landing, you can add some salt to it to make it more enjoyable, or just hang it up on a rack to dry. So many possibilities now...

#5. Money money money...
So in the Minecraft world, there is no economical value of these flesh strips (okay, accept for that crazy priest guy). Heck, there isn't even an economy! But in servers, there are, and some allow you to sell rotten flesh for whatever currency they have. You can use /sell or whatever command says that you can sell stuff in /help.