Stunning Abilities:

Gladiator/Paladin
Gladiators/paladin have this amazing skill called Shield Bash which have an amazing short recast of only 6 seconds

Marauder/Warrior
MRDs and WARs get Brutal Swing, sadly it has a recast of 30 seconds, which is quite long compared to the GLA/PLD stun.

As someone who plays a lot of different jobs and classes I can really recommend putting your stun on the same hotbar key that's easy to reach for you, that way things will become automatic and you won't even have to think about it.

Remember Stunning is teamplay:
Don't hesitate to ask your DPS to use stuns when you're on the cooldown timer.
LNC/DRG's have Leg Sweep and PUG/MNKs have Steel Peak

Hope this helps for you!