Level 65-80 (376-450 engineering)
Bombs - Saronite Bombs, 'nuff said. You could use explosive rabbits, but you need to have the EMP device on your belt to set them off. Portable Nuke for Goblin engineers, but you'll want to use a shield spell/item with it to offset the damage. It does have the highest bomb damage in the game, though.
Goggles - You will want to make the Mechanized Snow Goggles at 65, which will automatically make the armor type you can wear (Cloth,leather,mail,plate). Enchant them with the 45 stamina Mind Control enchant, and replace them at 72 with your class type epic goggles, which are ilvl 200. These will last you until 80, and even will be useful until you've gotten all your other armor up to the 226-232 level.
Flying Mount - At level 70, you can build your own epic flying mount, but you have to go back to Shadowmoon Valley to learn the recipe.
Trinkets - Noise Machine and Sonic Booster can be very useful for a level 65 tank, as well as any other class. I've been playing around with having my paladin use 1 of each while going thru BC dungeons, and they seem to work out well. I'm not sure if I'd want to use the Noise Machine on a Warrior Tank, since it might interfere with rage generation. There's the Gnomish Lightning Generator trinket, and if you make 2, the crit will stack, but you can't activate them individually. Also, I've found that if you only equip 1, and have the other in your bank/bags, you can't fire the equipped one with a macro. Either make only 1, or never use them seperately.
BTW, Gnomes can equip any of the 3 trinkets at level 50, since they have a +15 racial engineering bonus, they can get up to 390 engineering.
Have a level 420+ engineer make 2 Noise Machines for your level 50 mage, equip your Xtreme Lite boots with Greater Fortitude, your belt, and your goggles of choice, and you'd have at least 173 spellpower and a buttload of stamina and other bonuses.
Self Portal - Wormhole Generator: Northrend is awesome. Not only is using the flight taxi expensive in Northrend, it takes forever to get from Dalaran to Borean Tundra to do the fishing daily there. Not with this, you drop from the sky right into the lake, and are easily able to complete the quest within minutes of accepting it.
Cloak Enchants - Melee and caster varieties, you probably won't even think of getting another enchant for your cloaks.
Glove enchants - Haste, armor, or long range attack to finish off a fleeing foe, your pick.
Do-it-yourself Rocket Boots - Nitro Boosts 24 critical strike rating and blazing speed, hell yes!
Mailbox on demand - MOLL-E If you ever think of going for the Insane in the Membrane achievement, you would kick yourself for not having engineering to be able to turn in multiple librams to the Shen'dralar. Also useful for mailing off stuff to an auction house alt when your bags get too full while farming.
Robot Butler - Jeeves is really nice while in a dungeon to sell your trash too on your 20th run thru Stratholme for the Baron's mount. Or, your tank threw a drama fit and left during a heroic? Pull out your tanking equipment from the bank, and show the rest of the PUG what you can do.
Resurrection - And so much more! The Army Knife is everything you need for your tradeskills, plus you can rez your healer if you survived a wipe.
Land Mount - It's a motorcycle, and it always makes you feel like you're going fast, especially when it shifts into second. C'mon!
I'm not going to discuss ammo, scopes, or ranged weapons, since they don't have very much of a profit margin now, and ammo will be going away with Cataclysm.
I hope that some will be inspired to look more closely at engineering, and not just at the few enchants, port, butler, mailbox, and mounts that are useful at level 80.