Rawr is a great third-party app (it's open source, and doesn't interface with the game at all) that's designed to help you figure out everything you need to know about your character, including how to best buff, enchant, and gem up your gear and set up your stats for whatever you'd like to do. It started out as an app strictly for tanking druids, but recently, it's grown to include pretty much all classes and stats. Our good friends at sister site Download Squad just recently found out about it, and they've got a nice basic writeup posted that serves as a good overview if you've never used the app before.
They also mention that the app just got another update, to version 2.2.25, and all of the release notes are posted over on their site. They're also looking for help -- if you're a C# developer, or know one who can work on their rogue and hunter models, be sure to let them know. Since the program is open source, anybody who knows the code can jump in and add their own features, or fine-tune the class choices, which then helps out everyone who uses the software. Rawr is definitely an excellent resource for anyone looking to get their gear perfected -- it's a free download for Windows on their site right now.
Update: Since this post was written, the app has been updated to 2.2.27. That was fast!