VALVe just recently released two new mods on their platform Steam: Dystopia and Eternal Silence. Now, it's true that these were already there in the first place. However, they are now part of your games list. That means you can download them through steam, not needing to install anything.
Dystopia is a multiplayer total-conversion mod for Half-Life 2. Players fight through battles in the real world, all the while trying to gain access to jack-in terminals. After jacking in, guess what? You do it again. It promises constant updates and looks very fun.
Eternal Silence is a bit different. Basically, you fight it out with giant space-bound ships. Either go on foot to take out the enemy computer systems, or drop bombs from a fighter jet. In fact, the ships can be as large as 32768 cubic meters!
Both of these games are very fun and promising. Keep in mind, you need a source game (basically any game made by VALVe) to play these games. Have fun!
