1. Exploding Bunnies
The Personal Electromagnetic Pulse Generator has many uses, but perhaps the most entertaining is to explode an Explosive Decoy from range.
Don't have a shaman in your group for Auriaya? No problem. Leave a harmless little bunny instead! With a 30 yard range on the EMP, the tank can explode it just as she approaches. What's better than an exploding bunny?
The EMP will also stun most mechanical mobs in the game, which is helpful when running Gnomeregan, zapping clouds in the Geyser Fields of the Borean Tundra, as well as on several boss fights in Ulduar to deal with trash.
The bunny offers some creative ways to pull mobs as well, and you can even Misdirect targets to attack it, causing it to explode without the help of your EMP belt.
Saronite Bombs are extremely cheap to make, and will deal fire damage in a ten yard radius. They are instant cast, and you can throw them while running!
Tanks can use these to pull groups in the same way that Death Knights can use Death and Decay. From range, you are sending a quick burst of AoE damage into the group, giving you some aggro with each mob. Box of Bombs is the latest way to make these bombs, and each box yields 12-20.
Frost Grenades give a rooting effect, and are considered a freeze. This means that if you have a talent based on freezes, such as a Frost Mage's Shatter, this is probably the option for you.
This is usually considered a PvP grenade, and can be used to slow the masses encroaching upon a flag, flag bearer, or other target of interest. They are not usable in Arenas. For PvE, it can be helpful if adds get way out of hand in fights like Gluth and Thorim, although due to its 1 minute cooldown, it can't be relied upon solely for add control.
Cobalt Frag Bombs are similar to Frost Grenades, but apply an incapacitate effect rather than a freeze/root. They are the direct upgrades of the Adamantite Grenades, and offer more damage and a 0.5 second cast time, compared to the Adamantite's 1.0 second, although the Adamantite still works on level 80 targets. Despite the cast time, it can be used while running.
The Frag Belt is a great accessory to have if you start using Cobalt Frag Bombs on a regular basis. They are useful for interrupts, breaking a Rogue out of stealth, and keeping a melee class at range a bit longer. They are also fun to throw at healers.
In PvE, it would be useful for soloing. For example, Protection spec players could use it as an opener when working on groups to minimize initial damage.
Although the Global Thermal Sapper Charge is usable only by Goblin Engineers, it too has its merits. Once upon a time, this Charge, along with Saronite Bombs, were used to do decent siege damage in Wintergrasp and Strand of the Ancients. The damage was so decent, in fact, that when combined with a few other Engineering tricks, the guild Goon Squad was able to capture Wintergrasp in 62 seconds. Needless to say, the siege damage components were nerfed.
One thing to note about this item is that using it will hurt you, and everyone around you who is not an ally. One of the best uses for it is while AoE tanking. As long as you are getting sufficient heals, blowing this 5 minute cooldown can come in handy when you're really scrambling to establish solid AoE aggro. An undergeared tank, for example, might find this helpful, as would a weaker (or less AoE-oriented) off-tank when trying to peel mobs off of the main tank fighting the boss.
3. Potion Injectors
Yes, I'm aware that potions now stack to 20 naturally. But as an Engineer, using these injectors will actually boost the effectiveness of those neatly stacked potions, be they mana or health.
Doing this gives you 25% more health or mana upon consumption, and they do, according to Wowhead comments, stack with the Alchemist Stones (a group of Alchemist-only trinkets) effects, which increase the effectiveness of health and mana potions by 40%. Note: The Alchemist Stones effects do not stack with one another, so by wearing two trinkets you cannot get an 80% bonus effect and then stack your potions using injectors for a 105% total bonus.
4. Hand-Mounted Pyro Rocket
Haste junkies enchant their gloves with the Hyperspeed Accelerators, and some tanks might choose the Reticulated Armor Webbing, but what about DPS classes?
The Hand-Mounted Pyro Rocket enchant is actually an excellent choice. It no longer triggers the Global Cooldown, although it does share some cooldowns with some of the other Engineering goodies. It is on a 45 second cooldown, and will not reset your swing timer. To make the most of it, you could macro it, either on its own, such as "/use 10" or better yet, piggybacked onto an ability that you hit at least once every 45 seconds:
/cast Name of Spell
This is also an excellent tool for soloing as well as for tanks, although they would have to give up a tanking enchant to have it. It gives you yet another way to pull from range, and adds more DPS to your rotations than any other in-game enchant.