Some modders, like the creators of XCOM's The Long War, worked with Firaxis to have some mods for the game available day one. There's also an official co-op mod!
Why suffer the cruel dice rolls of XCOM alone when you can suffer with a friend? 10 months of work resulted in a two-player co-op mod, which the mod team plans to expand into a 12-player mode when code is approved by Firaxis. Read the short instructions on how to get the mod working with a friend here.
Long War SMG Pack
This official mod adds a submachine gun option for your soldiers. It allows them a greater range of movement and makes them harder for the enemy to detect, but comes with a cost: less damage and less ammo.
Long War Leader pack
It provides a new development path for your soldiers that allows them to obtain leadership perks, similar to the custom Officer class from the Long War mod. These perks result in buffs that will help the entire squad, but you can only take one of these leadership characters with you at a time.
Long War Alien Pack
The Long War gang replaced its Muton Centurion mod with this pack of new aliens. It includes more than a dozen new aliens to fight against, including the Muton Centurion and Elite, a new alien sharpshooter, a giant Chryssalid, and a bunch of new ADVENT troops. Cower in fear.
Long War Perk Pack
An official mod overhaul of the perk system that changes or adds 70 abilities and allows for picking between three perks per soldier level. The UI has also gotten some tweaks to make space for all these new options.
Tools and utilities
Mod Config Menu is a tool that some mods require you to use for them to play nicely together.
The mod editor, called ModBuddy, is a separate download of about 45 GB. If it sounds familiar, it's because it's the tool used for Civ 5's mod-making. Modbuddy can be downloaded as a Steam Tool, and with it you can debug your mods and upload them directly to Steam. The SDK provides 1,900 game script files, 2,400 maps, thousands of props, materials, and textures, plus hundreds of character assets and animation sets.
If you've clicked any of the links on this page, you know the Steam Workshop is the best place to find mods, and many can be installed with a single click. However, Nexus Mods also has a hub for XCOM 2, and the Nexus Mod Manager already supports the game, and is a great tool for downloading, activating, and managing your favorite mods.