There is a bukkit plugin called MineralVein.
It allows you to control the size, frequenty, height, density (1 big lump or spread out ores).
It also is posible te make them spawn in biomes of your choise or increase the frequenty in a certain biome.
If you want to you can also generate other blocks underground.
I for axemple have a 0.5% chance for ores to turn into their block version
so a gold ore can become a gold block. you can also have ores spawn together like coal near diamonds or you could indicate its precense with a certain stone type like cracked brick or prety much everything.
this also works with mod ores/blocks. this way i have a world where every thing is rare but if you find it you find +/- 300 of it depending on the ore
gold, diamonds and uranium are smaller groups.
Therefore i think this is an amazing plugin that allows you to do control ore generation on smp with a large amount of control.
I do have to warn its a bit hard to use in the begining but you can let it regenerate al the ores and delete the old one with a simple command.
It does this by reseting al ores with stone and than replacing the stone it doesent replace airblocks. and therefore keeps your world intact if you want to convert.
