This tool allows you to keep track of users, and communicate more easily with users and other mods. Reddit really should be pinging these guys to bake their toolbox functionality directly into reddit, but this is the second-best thing, and I applaud these guys for making a great extension.

If you use it once you'll never want to moderate on reddit without it. Its just that useful. And it has many customizations to get things just the way you want them.