One of the most common questions I see asked in guild chat is who can cut a certain gem or if anyone has a certain enchant. Back when Cataclysm came out, we actually had a stickied forum post with every gem cut and which guild member could cut them, which was very handy when trying to decide on pattern purchases.
However, did you know that you can actually view, browse and search the professions of every guild member, whether they're online or not?
1. Open guild tab
2. Select "Professions" from the dropdown menu at the top
3. Check the box at the bottom that says "Show Offline Members"
4. Find the profession you want, then hit [View All]
5. Browse, find whatever you're looking for, then hit "View Crafters"
6. A box will show up titled Guild Crafters, showing you who can make it!
So you see, there's really no need to ask in guild who can craft you something, or who had this cut, or etc. Just open the guild tab and five seconds later, you have your answer! All the usual browsing methods work while filtering your search - you can search by name, you can type in Intellect or Inferno Ruby, or you can manually select which slot you want to browse.
Very straightforward, but I suspect many people have no idea you can even do this. I only chanced upon this sometime in Firelands, I think it was?
This is a very convenient way to know which of your guildies can craft a specific item you're looking for. However, it can also be a nice way of finding pieces for low-level alts, or for Transmogrifying! Wondering if there's a nice robe available for your lowbie Mage? Check out guild Tailoring! Looking for a Plate Helm to round out your Transmog set? Select guild Blacksmithing and browse away to your heart's content!
(Funny story: I did this to browse which guns our guild Engineers could make, as I wanted to transmog the Mark "S" Boomstick. I was delighted to discover that someone could make it! ME. Yeah. I had completely forgotten I could make it and had just assumed that I couldn't.)
There's no guarantee you'll find what you're looking for, but I'm often pleasantly surprised at the random old-world stuff my guildies can make! Of course, if you're like me, you'll find an awesome, no-longer-available weapon that can only be made by someone you've never heard of who hasn't logged on in over 3 years.
(Don't forget that Mail pieces can be made by both Leatherworking and Blacksmithing, and that there are a few craftable non-Cloth cloaks.)