Online multiplayer is the theme of the latest Humble Bundle.
The Humble Gamepedia Bundle includes a stack of content related to online multiplayer titles that happen to have official wiki pages hosted by Gamepedia. Many of the titles are free-to-play, so what you're mostly buying is add-ons and boosters.
Pay what you want over $1 for Awesomenauts, Heroes & Generals (Early Access) - Weekend Warrior Pack, Dirty Bomb - The Ultimate Starter Pack, WildStar — Jumpstart Pack and a 30-day Curse Premium subscription.
Cough up more than $5 to also receive a Vertiginous Golf 4-Pack, SMITE - Freya + Pixel Buster Skin, Neverwinter - Humble Pirate Pack, War of the Vikings and Robocraft (Early Access) - Twin Robot Pack.
The premium $15 tier offers Dirty Bomb - Humble Bundle Loadout Cards and Case, Path of Exile - Arctic Crown + Cobra Pet and Life is Feudal: Your Own.
Also, Stronghold Kingdoms is free to anyone, whether they buy in or not. Just visit Humble Bundle to take advantage of one or both offers.