What is this 'Autoupdate' app that keeps popping up in the dock?

That icon is from http://sparkle-project.org, which is an open-source auto-updating framework. However, it could be embedded in almost any application, since Sparkle is one of the most popular auto-update frameworks in indie Mac software.

Did you recently install new software that has Automatically check for updates checked in the preferences?

Edit to include my comment.
If you're interested in locating which application it is from, you can hold ⌘ Command and click the icon in the dock (or right-click and select Options > Show In Finder), and it will open its location in the Finder. You can then right click on the top of the Finder window that appears (on the icon), to show the path to the containing application.

With this method, you can see that this Autoupdate.app is from within iTerm.

If you right click the Dock icon and select Options > Show in Finder, you should be able to see the path to the application it resides in. (If necessary enable the "path bar" in Finder's View menu ⌥⌘P)

You can also use the Activity Monitor app to find that app and in the details, it will show all the open files. With that information you should be able to suss out which program / package is updating and turn off the update check or just update it directly from the developer site.