Launchpad is an open-source game launcher written on C#. It's free, available for commercial and noncommerical use, and provides all the baseline features that a game launcher should have.
It runs on Unix and Windows, and currently handles installing, updating and verifying your game. It can also display a changelog.
The source code is available here:
Binary builds are available through the GitHub wiki or on the Unreal Engine forums:
Plans & Pricing