Modrinth
Modrinth is a platform for distributing Modded Minecraft content.
The Website
On Modrinth's website, you can discover Mods, ModPacks, Shaders, Data Packs, Resource Packs and Server Plugins. Their search tool is great for finding what you need.
The Modded Launcher
Modrinth has a Modded MC Launcher, which is a good choice for playing Modded Minecraft if you are only interested in using Mods / ModPacks from the Modrinth platform, due to its lack of support for CurseForge content.
If you want to use Mods / ModPacks from either platform, there are other Launchers that support both platforms. See the Getting Started section.
Modrinth Servers
Modrinth supports Modded MC Server hosting on their website.
Interesting info
The most downloaded mod on Modrinth is Fabric API. This Mod is used by almost all Fabric mods, and thus is always downloaded alongside them, hence the high download count.
The most downloaded ModPack on Modrinth is Fabulously Optimized.
As of writing, Modrinth hosts 30k+ Minecraft Mods.
Modrinth supports Mods that use LiteLoader, Risugami's Mod Loader, and Rift.