Gnomish Army Knives have been a long time go-to item of mine. If you are a gnomish engineer you are set, you can craft these for 20-30g. If you are not, watch for these items to come up AH and snipe them within the same price range. Many posters seriously underestimate the value and market of these items and you will often see them posted at ridiculously low prices.
On my server however these will sell nicely between 80-125g each. The best thing about them is that they are used by five different professions. (and they act as jumper cables if you are an engineer).
So the mat's required for these are:
|10||[Saronite Bar]||1g 50s||15g 0s|
And they can be used by Skinners, Miners, Cooks, Engineers and Blacksmiths! That is a pretty huge audience. The items this knife replaces are: Gyromatic Micro-Adjustor, Arclight Spanner, Flint and Tinder, Blacksmithing Hammer, Mining Pick, and a Skinning Knife. Not bad for bag space saving if you share even 1 of these professions with another, it will save space in your bags. Which is at a premium usually.
Are there other items like this you can recommend that have a huge audience of uses?