AuctionLink allows you to search for items in the AH by shift-clicking them. It's always a pain to search for some item with a weird/long name, and think there are none on the AH because you spelled it wrong. Now you can insert the name into the search box just by shift-clicking it as you would link an item.
You can alt shift click items such as "Aquamarine Ring of Shadow Resistance" to just search for "Aquamarine Ring".
It works with your bags, trade skills, auction frame and inventory, and also links in the chat channel.