The Launcher powerup is used to shoot forward at a high velocity. You have very little control while using this powerup and it's not a good idea to use it while descending. The best times to use it are when the enemy in front has thrown an ice powerup or slime minion powerup down.
This powerup by default is a little overpowered, so it's a good idea to nerf its settings a bit unless you have a server which you anticipate to have 8 people on at once. The concept is simple. You right-click and it lets you speed forward.
To use the TNT powerup, you need to either right or left-click ahead. If you left-click, a block of primed TNT will fly ahead. If you right-click, it will be dropped at your location.
When the player right-clicks with this item in hand, an arrow will launch forward and target the nearest enemy. While it doesn't deal a lot of damage, it can sometimes push them into other obstacles so you can run ahead.
It is highly recommended you disable this powerup for now.
The Thief powerup doesn't really work yet. The idea behind it is when you right-click another player, you will take whatever item they currently have for yourself. If they have nothing, then it doesn't work and breaks.
There are two ways to use the Ice powerup. Much like the TNT, it can be thrown forward, or dropped behind. It should be noted that enemies can recover it during the time that it takes to detonate. Once it explodes, it creates an ice sphere that encases anyone inside for a period of time and/or blocks the path ahead.
Slime Minions Powerup
Similarly to the Arrow powerup, the slimes spawned by right-clicking will not harm your opponent physically. With that said, they form a sort of blockade that can be difficult to get through. This would be a good enemy to use a launcher to get over!
None yet! If you're a developer who has made any powerups, send me a message of some sort with the link to the plugin's BukkitDev page and I'll list it here.