There are a couple of downloads every Jazz Jackrabbit player should obtain. At the moment there is only one download that is mandatory to make the online party mode work. That download is the 1.23 game patch. Other downloads on this page can be useful additions.
A community-created add-on/patch for Jazz Jackrabbit 2, JJ2+, fixes many bugs and adds many features to the game. It works for 1.23 and 1.24.
Note: if you get error messages about version when running the patch, try if the following forum topic clears things up: Versions of Jazz Jackrabbit 2, and why patches don’t work.
Note 1: The game is able to effectively host servers without the need for any of these programs to be run, but many people use them due to the very useful features they provide. These are ONLY meant for hosting games.
Note 2: Most of the useful features that these programs provide are already built into JJ2+ (see above). Unlike JJ2+, these programs must be run along-side JJ2. Still, some people might prefer them.
Eat your lima beans, Johnny.