Most professions have some crafts that aren't very useful, but mostly for fun. With engineering this is rather the rule than the exception. Although there are plenty of very useful crafts as engineer, I still feel that most of them are tinkery fun stuff. Blizzard has really succeeded in making you feel like a crazy scientist when you're an engineer!
One thing that is just hilarious to do when you're an engineer is blasting lowbies with Decoys. Ok I realize I'm letting out my evil side here, but I just can't help it. It's really entertaining. I like it so much I'd even find it slightly ok if someone did it to my lowbie. It's an acceptable way to kill someone!
You need three things to make this work, and yes unfortunately, this trick only works for engineers.
1. The Decoy
2. The Trigger
3. The Lowbie
The first two are pretty easy. The decoy is called Explosive Decoy, and is a little bunny jumping around. It explodes when hit. Just placing it near lowbies might actually make them attack it and thus kill themselves, that is if they're stupid. This is of course the best way to use a decoy! Unfortunately, lowbies aren't as stupid as they look. So you've placed the decoy but the lowbie won't hit it? No fear! You can set it off yourself >:) By setting an EMP to your belt and pressing it the Decoy will go off instantly, dealing damage to any hostile target around.
Finding a proper lowbie might be a little more difficult. The Decoy does about 1600 damage, so you'll need someone with less hp than that, usually someone below level 40. Booty Bay, and sometimes Gadgetzan are good places to look.
But wait a sec, you'll probably say. Won't the guards shred me to bits if I start blasting lowbies? No! And that's the best part! Since you're not actually the one doing the damage, the decoy is, the guards couldn't care less! So off you go making BB or Gadgetzan unsafe, if you're lucky (and smart enough) to be an engineer!