Back in the days before he glued a chainsaw to a machine gun, Cliff Bleszinksi was best known as the guy who helped create the green, over-caffeinated lagomorph with a gun named Jazz Jackrabbit. Jazz debuted in 1994, and then came back with a sequel in 1998; ten years later, the Jazz Jackrabbit 2 Plus mod came out, adding "countless features and bug fixes" to the game. It's been updated several times since, most recently this past weekend, which saw it move to version 5.0.
It's impressive that a game this old could still have this much done to it, but it's been awhile since the last update, and this one is pretty ambitious. Version 5.0 adds new mutators that enable "nearly unlimited" customization of characters, weapons, mods, and rules, plus a new multiplayer game mode, improvements to the Treasure Hunt and Race modes, a better internal level browser, new sprites and animations, and a number of other things.
It's been an awfully long time since I played Jazz Jackrabbit, and based on what I recall of it, things have changed quite a bit. Back then, it was a rather Sonic-like platformer, but now, going by the description on ModDB, it's "the world's premier 2D Arena Shooter."
"Players can clash together across fifteen different multiplayer game modes (some team-based, most combat-based) with ten different major weapons (all but one of which can be powered up to deal more damage and use different graphics and/or movement), or else sit out a round in JJ2+'s exclusive spectator mode," it states. "Play for fun or in competitive ladders and scheduled events, on your own or in a clan. Use the traditional weapon system, or turn on Mouse Aim to take extra control of your gunplay and take down your opponents with perfect shots."
Jazz Jackrabbit 2 Plus requires a registered version of Jazz Jackrabbit 2 to run. Once you've got that installed, grab the mod here.