Nexus Mods 148 games Sign in with your personal API key. Premium accounts download inside Mortar; free accounts use Nexus's own Mod Manager Download button, which the browser extension or Mortar's link handler hands to Mortar.
CurseForge 8 games No account needed. Search and install from inside Mortar.
GitHub 5 games Add installs a mod's latest release. Nobody reviews what is published there, so the first download says so once.
Thunderstore 306 games No account needed. Mods install with every dependency they name, and r2modman and Gale codes and .r2z files open in Mortar.
Vintage Story Mod DB 1 game Vintage Story's own mod site. Mortar fetches the mods each one requires, and the Mod DB's 1-click install button can open in Mortar.
Paradox Mods 1 game No account needed. Paradox's own mod site for Cities: Skylines II. Mortar installs these mods into the game's local Mods folder for each profile and leaves the mods the game itself subscribes to alone.
itch.io 1 game Needs your own itch.io API key, kept in your system keyring. Add takes the first upload that is not a demo or soundtrack.
Patreon 1 game Paste a Patreon post in Add mod: it opens in your browser, and the file you save from it is offered and recorded under that post.

The one exception to downloading from the source: a profile sent between two of your own paired computers carries its mod files. How pairing works