I created this build, like most builds, over time. Amumu is my main tank. I've owned him for quite some time and I managed to create a pretty decent build in my opinion. This is my first build so I won't get too far into it.

The runes I run are not these. Since I don't play too often I can never get too many runes. I usually run full ap quints, seals, and glyphs. Leaving magic pen marks. I chose this time to get a more tankier build.

I run the usually jungle tank masteries. Obvious points in the defense tree and I also get points in mana regen and reduction in time spent dead. I get mana regen because amumu tends to run oom quickly if you want to jungle efficiently without blue buff. I put points into Good Hands because since you are the tank. You can't be dead when your team is about to enter the team fight and also in similar situations.

Alright, this is where I think my build is the strongest. It has a pretty even ratio of health, armor and magic resist throughout the game. You would also be able to do some damage. I've noticed in my games that the aoe damage of despair, tantrum, sunfire cape, and curse of the sad mummy adds up. I would often do the most damage in my team.
The abyssal scepter drops the magic resist of all surrounding enemies by 20. Along with your basic attacks, the magic resist of a champion you are attacking is now dropped even further. As long as you are working with the ap caster, you can take out a prime target almost instantly.