Minecraft how to detect when a player accesses an anvil

I figured out how to do this. I use the stat.damageDealt scoreboard objective. First I run the following command once:

/scoreboard objectives add hit stat.damageDealt

Then on the 20Hz clock I do the following:

• First command block:

  /tellraw @a[score_hit_min=1] "Nice Hit!"

• Second command block:

  /scoreboard players reset @a[score_hit_min=1] hit

How this works is that points is added to the stat.damageDealt objective each time they hit/attack/hurt an entity or player. Then it runs the command on every player that has hurt another entity or player. When that command is done, the objective gets reset.

How I found this is that on the statistics menu from the pause menu, there is a statistic called Damage Dealt.

EDIT:

In Minecraft 1.9, use the repeating command block instead of a 20Hz clock.

You can't ignite the TNT in the Zombie's head slot, but you can use the Passengers tag to summon the Zombie with PrimedTnt riding it.

Example:

/summon Zombie ~ ~ ~5 {Passengers:[{id:"PrimedTnt", tag:{Fuse:100}}]}  

Since the Passengers tag is only really a flag that the /summon command looks for, /entitydata and similar commands won't be able to place a PrimedTnt on the Zombie. This means that if you want a PrimedTnt entity on the Zombie, the only real practical solution is to summon him with the TNT.

If you want a proximity trigger, you could summon a PrimedTnt with a fuse time of 0 at Zombies near Players.

Example:

/execute @e[type=Player] ~ ~ ~ execute @e[type=Zombie,r=1]  ~ ~ ~ summon PrimedTnt ~ ~ ~ {Fuse:0}  

We use @e[type=Player] instead of @a to avoid targeting dead players: we likely don't want a Zombie to explode because it's trying to walk over a corpse.