Bukkit

Alpha 0.1:

Current Features: -Build a predetermined missile silo made of 9 Diamond Blocks, 40 Iron Blocks, 9 TNT, 18 Obsidian and other miscellaneous materials. -Fire an explosive round at any coordinate with a slight delay. Rounds consist of 5 gunpowder, 3 Diamond Blocks and 1 TNT. -Easy to use system of setting coordinates. Two numbers written on lines 1 and 2 of a sign on the cannon (X and Z coords). -Help messages to aid you in pinpointing the problems with your cannon or coordinates.

Careful not to use "trapping chests" in creative mode for the cannon (it won't work). Also check that your cannon is 10 blocks high when your are done. Generally just make sure that you have built the cannon correctly. The plugin will try to tell you what is wrong, but it can be quite vague.