I have written many Java plugins for minecraft servers. I have also decompiled and modified the minecraft server and client programs. Most of this has been to create interesting game modes for the Minecraft PVP UHC gamemode. These plugins are for servers running 1.8 or earlier I would say. Your mileage may vary.
Mole 31,08,22 Top
The Mole plugin is probably my most well known plugin. It's old, but people still ask for it. Anyway, make teams, one player per team is actually a mole trying to kill their team, the moles are all on a seperate team.

Download it here
Cryophobia 31,08,22 Top
A plugin for the Phobia UHC recorded round. When activated the world slowly fills from the bottom up with ice, filling all empty space as it does. This was a technically challenging plugin to write that turned out fantastic.

Download it here
PyroPhobia 31,08,22 Top
A plugin for the Phobia UHC recorded round. When activated the world slowly fills from the bottom up with lava, filling all empty space as it does. I believe there is also a post processor that lets you replace all water on the map with lava.

Download it here
THS3 31,08,22 Top
I wrote a modded server that changes how the world generated, with amplified, normal and flat terrain alternating in bands. Also any type of mob dungeon can appear. It was originally for an SMP server but people wanted to play UHC on it and then it went from there.

Download it here
Shared Health 31,08,22 Top
A plugin that lets you team players and then gives them a shared health pool. So for example if one person on a To4 takes 2 hearts damage in reality each player takes half a heart. Absolute crazy fun when a teammate goes and does something silly like getting poisoned.

Download it here
Astrophobia 31,08,22 Top
Another Phobia plugin. I cannot remember all the details but i'm fairly sure it involved TNT launching from the sky along with 'shooting stars' and other things like that. I seem to remember it wasn't well balanced because we made the TNT increase towards 0,0 and the meetup was basically everyone getting blown up.

Download it here
Kings 31,08,22 Top
Kings is basically a plugin to make even teams and select a player from each team to be the king. The king has resistance and better health and attack BUT if they die their whole team dies. We did this for a WMC round and the final battle was like, 2 hours of people running in circles not wanting to fight.

Download it here
Ore Logger 31,08,22 Top
A plugin that records every instance of diamond and gold mined by a player. After the match you can 'replay' the veins mined and the plugin will copy the mined area from the game map and place it side by side with the original chunk. This lets you find people who are strip mining or using x-ray because they will have lots of chunks where the vein they found was not in an exposed cave. I caught a few cheaters with this, was fun.

Download it here