Pretty simple mod, honestly. It'll edit the multijump function to allow you to jump forever.
If you do not know how to install it, well I'm glad you asked, Little Timmy. To install, just have any other multiplayer mod with a tech folder, unzip the compressed file you get, and just place the cool file, the one named jump, inside the tech folder. Congratulations, you win at life, Tim. HAHA JUST KIDDING! It's standalone now, Tim Tim. You're safe.