This is a fairly large and robust addon written by Thaoky. It will keep track of virtually every stat related to your characters as well as inventory tracking. If you're looking for something that's been packed away on some random alt, you don't need to dig through each toon's bank in order to find it. All you do is pop over to the search tab and check Altoholic's database of your stuff.
What it does right...
The inventory management and tracking is great. As shown in the gallery, I can search for cardinal rubies and find out which toons have spare ones, if there are any in the guild bank, and even the transmute from my alchemist shows up to let me know I could make them. It adds a few lines to item tooltips again letting you know if you have any of that item on any other characters. This is handy when going through a recipe for a profession as you can figure out what characters have the items you need.
The profession, skill, and reputation tracking is also really nice. With it, I can go through the available recipes on other characters before hitting the auction house for materials. I can also keep track of which toons are progressing their professions at a decent rate and which ones need some cash or mats to get them caught back up. The interface will even give suggestions of what to work on next at that level for each profession.
If you're in a guild with people who have a lot of alts, it can be a pain to keep track of them at times. Altoholic can talk to other players who also run Altoholic and let you know who their alts are. It will also keep track of their professions so that you can bug them for enchants or glyphs when they're online.
Where it needs work...
Its tracking for comparing achievements, mounts, and vanity pets need some work. It has each of your characters side-by-side across the top with the mount names down the left hand side and icons for that item under each column. It's a lot like trying to read through a graphical spreadsheet with icons being lit up or grayed out. As a colorblind player, it was difficult to see the comparisons at times and could be frustrating if trying to quickly find something. Also, the achievements in the interface were in alphabetical order which caused things like 100, 1000, and 250 Quests Completed to be in a rather awkward series instead of the numerical progression of 100, 250, 500, 1000.
Let me explain. No, there is too much. Let me sum up...
Overall, this is an invaluable addon for anyone trying to juggle multiple characters. The ability to reference your other toon's equipment, inventories, and professions at the click of a mouse can save a lot of time and hassle. If you're one of those crazy people with a guild for your bank alt, then you'll be able to keep track of those extra bank spaces much easier as well. For as many features as it offers, it actually has a relatively small footprint taking up about a megabyte and a half of memory for the main addon. I know I'll be using it more in the future.
Where to get it: [Curse] [WoWInterface]