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.
Plus brought a whole new era to JJ2. Aside from tons of multiplayer fixes, multiplayer commands, server administration, mouseaim, and added gamemodes and features (phew!), Plus makes what was once hard coded into the game flexibly programmable through AngelScript! Basically, with the right experience you can now do whatever you want with the game. Your levels aren't limited anymore by the original game's events, you can alter their properties as far as your imagination goes!
This mod is the best thing that could've happened to an 1998 game, ever!
Check Jazz2Online.com and it's forum JCF to find downloads utilizing Plus features and to get help with scripting.