Nexus Mods has a massive and highly enthusiastic community-there are over 15 million members who have collectively downloaded nearly 3 billion files. Once installed, it'll even detect which games you own that it supports, making it easy to get started with modding them. Nexus Mods has its own mod manager called Vortex, which helps you manage the mods you download, makes installation much easier, and lets you know when there are new versions available for updating. 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.
