You need to create a free account to download mods, but if you're into modding it's absolutely worth it. You'll also find lots of mods for series like The Witcher, Mount & Blade, Mass Effect, XCOM, and many others. The Nexus Mods site hosts both simple and complex mods for over 600 games, probably most notably for Bethesda RPGs Skyrim, Oblivion, and the Fallout series.
The simplicity of the Steam Workshop is great, but when mods are bigger and more complex they may need more than a single click installation.
