Before you will flame - yes, I searched if it could be a repost. Found only some minor posts that You can salvage mechanicals in Ulduar for Chopper parts etc. Anyway... Engineering, even if doesn't look like a profitable profession, can offer quite a good gold too.
My main char, is paladin with maxed engineering and mining. After some time spent at searching for something that could be profitable, I came up with few things.
Low levels
Starting from lower levels - the bronze tubes are something what shine. Especially if there are non on the AH. The tubes are used in some low level quests, and the mats are ridiculously easy to farm or even buy from AH. Just get the mats, make the tubes and put it on AH. They sells for 2-9g on my server, while generally ,mats cost me like 50s-1g.
Laaateeerr....
Anyway...real fun starts at higher levels, as usual. Now,when we have Insane in the Membrane Achievement, some people (the insane ones) will go crazy, to get required reputation with that factions. As you can see, one of them is Darkmoon Faire. One of the ways to get reputation with that faction, is turning in Thorium Widgets - you can get a repeatable quest. It only works to Friendly, but still, a lot of people will buy the widgets, just to get their rep up. The mats, arent that bad to get - 3 thorium bars and 1 piece of runecloth. On my server, I used to sell them for 10-20g per one, depends if there are any already on AH - if there are, and they are cheaper, i buy them out, and resale at my price.
Early WOTLK recipes...
WOTLK came in with new engineering recipes...and once again, we have a new 'bronze tube' which is... Overcharged Capacitor - this item is needed for 2 quests, one is in Borean Tundra, so a lot of people will be doing it, even only to get BT quest achiev. Other one is engineer's only quest in Storm Peaks, to get scrap bot schematic, which i'll explain later. Anyway - once again, mats to make this item are REALLY cheap, and easy to get, now when cobalt bars are like 20-25g a stack, you can make a nice profit while leveling your engineering skill! The recipe requires 4 cobalt bars, and 1 piece of crystallized earth which you should have many ,just from leveling a mining - if You dont, buy eternal earth from AH, and shatter it into crystallized. The total mats cost is about 5-10g, depends on prices on your server, and im selling capacitors for 15-20g. Easy, nice profit. And believe me, people are buying them.
Scopes!
Then..the problem started at 445 engineering. Only 2 recipes (not including epic goggles) on which you can skill up. One of them is gnomish army knife, which is useless for most people, and basically is not worth to sell - going for like 1g on AH on my server, while mats costs about 15g (10 saronite bars + mining pick, skinning knife, blacksmith hammer). While it is CHEAPEST way to get 450 engi, even if you have to make tons of them...there is an alternative.. and that is....Heartseeker Scope. It requires 10 saronite bars + 2 twillight opals. So... depending on prices on your server , you could loose on it, or make a profit, I usually earn like 5-10g on 1 scope, and people are buying them, since its like best scope that hunters can get. Mats are like 35-40g, im selling then for 50g. So.. a nice profit, while getting these last, damn hard to get skill points.
Get to the Choppa!
The chopper is somewhat tricky. Since some of the mats are engineer only, and the skill is needed even to buy the mats (or you can get some from AH if some engineers got it from Ulduar). Anyway... If someone is serious about getting the chopper, tell them to give You the money for engi-only mats, rest of the mats, and a tip. Now, while it being a rare, and expensive recipe, put on your tip. I charge like 200-500g just for making it. And made like 4 of them already. Salvaging the mechanicals in Ulduar (Leviathan ,Deconstructor, Mimiron) is also nice source of cash, but that already has been stated.
Basically, all of these tips depends on your server ,really ,was lucky, since they work on mine (Aerie Peak EU, alliance). Go on and try it yourself.
Scrapbot
Aaaaand now. Here is something that is totally common sense, but some engineers may do not know about that, especially if they skipped Storm Peaks while leveling. In the area south of Ulduar, called Inventor's Library, You will find a lot of mecha gnomes. Kill them, and You will get an item starting a quest - Scrap-E. Accept it, and get the needed mats - 4 handful of cobalt bars, 5 overcharged capacitors, and the card itself. Go into the library, turn in the quest, and You will learn schematic to make scrapbot, which is a WOTLK version of repair bot, and now, it also sells the reagents. The mats to make it are cheap aswell - 10 saronite bars, so its like only 15g for making that lil bot which is really useful while grinding instances, or raiding.