I don't know about you guys, but I have five characters with some 12+ professions between them. Sometimes it's very hard to keep track of who knows what. In the course of my mod research, I found this nifty thing call Professions Book. This mod will scan all of your crafting recipes and then when you visit the Auction House, it will color code the listings so you can easily see which recipes your toons know and which ones they don't.
I personally find this quite handy because I have a tendency to shop on my bank toons and not my mains. It gets annoying to remember what recipes my Jewelcrafter still needs when I'm shopping from her banker. With this new mod, not only was I able to easily see which recipes my JC doesn't have yet, but I could also shop all the other professions from the same toon. Even if you don't use bank toons to shop like I do, this mod still gives you an easy way to shop for all your toons using whoever happens to be at the auction.
Maybe I'm the only weirdo who shops from the wrong character, but sometimes you spot a really good deals during random auction checks. Most of the time I am not using the toon in question when I see a recipe worth considering. If the price is really low, I might switch over to the proper toon to check. But usually I do nothing because I can't be bothered with the hassle. I think you could probably accomplish something similar using other crafting tools that let you query your recipes, but I found it much more convenient to have the auction house listings color-coded for me.
I give Professions Book a solid "thumbs up"!