So it's been a while since I highlighted a mod, but I feel this one deserves a mention purely because I really want to try it out myself.
YellowAfterlife, the creator of the unofficial mod for Vlambeer's Nuclear Throne, has spent a lot of time and effort reverse engineering the inner workings to create a mod that allows for online multiplayer between yourself and an online buddy over Steam. And that's pretty exciting. It also includes some bug fixes and a replay mode to record runs which works outside of the co-op parts as well.
For those unaware, Nuclear Throne is a very fast-paced rogue-like shoot em up set in a nuclear apocalypse landscape. You choose 'mutation' upgrades after accumulating enough pickups and you use whatever weapons spawn for you as well as whatever ammo does. It's addicting as hell, and the idea of playing with a friend as well is a very appealing one.
Although the mod is not developed by Vlambeer themselves, the creator has been in contact with Rami Ismail who has been on a legal quest to see if releasing the mod would even be allowed. And it seems that the answer was yes, fortunately.
If you're as interested as me, go check out the blog entry for Nuclear Throne Together here. I meanwhile am going to see if anyone on my Steam List is interested in two-teaming the wastelands with me.