Jazz Jackrabbit 2 Plus (JJ2+) adds countless features and bug fixes to both Single Player and Multiplayer gameplay within JJ2, and is used almost universally by most serious players. It has much better network code than the original game, adding console commands to online games, fixing most major game-crashing errors and connection problems, allowing permitted clients to log in as "remote administrators" in someone else's server and control the gameplay, and adding several new multiplayer gamemodes. It additionally fixes or enhances many of the events in the game, and allows level designers far more power than in regular JJ2 through the addition of an easy-to-use scripting language. JJ2+ takes everything great about JJ2 and adds so much more on top of it; it is truly a must-download. JJ2+ is distributed under the MIT license.
NOTE: This is an OUTDATED VERSION of JJ2+. Look elsewhere in this downloads section for the latest revision! For the first time, JJ2+ is now being distributed as an installer .exe instead of a bunch of random files that you need to figure out for yourself. The installer will also do some handy stuff for you like registering context menu (=right click) actions for your .j2l files in Windows Explorer. There's also some network code changes, some fixes to bugs that made certain servers unjoinable, a less obnoxious spectator mode overlay, and a bunch of other stuff. Check it out, stay updated!
As ever, for full changelog and installation instructions, please consult the readme located in your download or online.