Unread XKCD is a free Chrome add-on developed by Jeffrey Meng from BrownieInMotion. This add-on falls under the category of Browsers and specifically the subcategory of Add-ons & Tools.

Unread XKCD enhances your xkcd browsing experience by adding a "Random Unread" button to the xkcd website. With this button, you can easily view a random xkcd comic that you have never seen before.

It is worth noting that the add-on has limited access to your past history and can only ensure that when you click the "Random Unread" button, you won't be shown any xkcd comics that you have seen since installing the extension. The add-on can optionally scan your browser history from the past 90 days through its settings, but it cannot access data prior to that due to limitations imposed by Chrome.