Enderal, a total conversion mod of Skyrim, is about to make the jump to Steam. Soon players will be able to play the lengthy campaign for free without fiddling with their Skyrim directory.
Enderal is a Skyrim mod that's been in development for years. I've been playing for hours, and I feel like I've barely scratched the surface.
As a total conversion mod, Enderal is an entire game built on top of Skyrim that has a 20-hour expansion if the 50+ hours of new gameplay isn't enough for you. The mod makes use of the Skyrim engine and some assets but is otherwise a wholly separate and unique experience.
People who wanted to play the award-winning mod have had to install the mod over their Skyrim installation, not allowing players to have access to both games at once. The new Steam version of Enderal will let you have a separate installation of Enderal so that you don't have to bounce back and forth between Skyrim and Enderal.
The massive total conversion mod for Skyrim, Enderal, is getting DLC that will add 10 to 20 hours of quest content. It's scheduled to arrive sometime in 2017.
If you can't wait for the Steam release, you can install Enderal now over a legal copy of Skyrim.