Zytron II
Developer: Trinosis
Genre: Shooter
Available On: PC (via Desura)
Co-Op Mode: Local (4 players)
Price: $7.99
Demo w/ Co-Op Available: Yes 
I remember one summer day when I went over to my friend Pete's house to play some Nintendo with his brothers and him. The two most popular games (for us) at the time were Gradius on the Commodore 64 [Ed. Note: to this day, I have yet to meet anyone with as vast a collection of Commodore 64 games as Pete's family] and Life Force on the NES. The reason for this popularity was fairly simple - there were two of us altogether and as we couldn't play the same game at the same time, two of us could and rotate out. I've played a fair share of indie games over the past couple years, and it never really struck me until recently when I played Trinosis' Zytron II how few solid horizontal side-scrolling shooters are available out there. So many shooters these days are geared toward the twin-stick/zombie side of things, so happening upon Zytron II while searching for co-op indie games has proven to be a most welcome discovery.
The backstory to the game is... well, quite long, actually - 2,600 some odd words long, to be more-or-less precise. It's rather amazing that the one-man development team at Trinosis, Kevin Murphy (not that one), spent the time putting together a backstory and then writing it all down with accompanying artwork. I bring this one point up as it's rare in this day and age to see such a thing for this particular genre. It is far more common for a brief paragraph intro that riffs off of the idea of "These <alien/zombie/creature/advertising banner> jerks are attacking, so go kill them." What seems like decades ago (because it was), it wasn't uncommon for a game to have a whole page or two in the game's manual dedicated to providing the player with all of that information. The dedication to truly try and recapture that feeling is evident even here. Anyways, the short version of all that backstory is that a group of military fighters get pulled into a wormhole where they discover some unidentified spacecraft with non-friendly intentions. So with that in mind, let's get down to the "go kill them" part.
Zytron II's gameplay is what one likely expects from a side-scrolling shooter. The game slowly progresses through a level with various types of enemies popping up along the way to try and shoot you down while you dodge their bullets and return the favor (ideally, to a much a greater success than they). Fortunately, you have a shield of energy to help protect you from all of that incoming fire, an upgradeable (to an extent) weapon system to deliver more firepower, and the occasional "kill everything on the screen" bomb to help see you through it all. The shields surrounding your ship keep you from dying the first time you're hit with a stray enemy bullet, and can be regenerated with crystals that are scattered throughout the level and randomly dropped by foes. These same crystals also upgrade your weapons, in a way, by enabling your ship to fire a few bullets to your side and rear as well as increasing the amount of bullets fired dead ahead. The game is broken up into 32 levels that, thankfully, don't need to be completed all in one sitting. Each level probably takes around 10 minutes to complete (depending on how quickly you finish off bosses), so the game is perfect for brief "sit down and play" co-op sessions.
Speaking of co-op gameplay, having a few friends join in on the hail of bullets and mayhem is easy. Zytron II allows for complete drop-in/drop-out co-op during a level, though a player would need to configure his/her controls prior to doing so. The game supports keyboard, mouse, and gamepad inputs so you're certainly not without options. Additionally, a player is free to enter his/her own name in and have his/her achievements, level progression, and high scores all saved so if he/she feels like flying solo every so often, they are absolutely able to do so. In keeping with the whole co-op theme, the game also features an option (turned on by default) that allows players to share their shield energy with one another simply by getting close to one another. In other words, Zytron II is a co-op game through and through.
While I may be able to wrap this whole thing up with some kind of punny or "ho hum" recap, I feel that perhaps the best course is actually to allow Kevin to summarize things, courtesy of an interview with Joystiq:
If you want an old-school scrolling shoot-em-up for the PC that's brought up-to-date with modern multiplayer gameplay, then Zytron II is the game for you
Wrap-Up
The Co-Op Experience: Up to 4 players may play together locally to shoot down as many alien craft as possible across 32 levels. Players may recharge one another's shields by coming into close proximity with each other.
Zytron II is For: Anyone looking for a fun side-scrolling shooter to play with friends