Very easy guide to follow, very useful for PvM based servers, enjoy. Any problems feel free to post here or send me a PM, - Stunz.

create RealDrop.java

    package server.model.npcs;



/*Author Scott*/



public class RealDrop {

	

	public int item;

	public int amount;

	

	public RealDrop(int item, int amount) {

		this.item = item;

		this.amount = amount;

	}

}

NPCDrops.java (replace all)

    package server.model.npcs;



import java.util.*;

import java.io.*;

import server.model.npcs.RealDrop;

import server.util.Misc;



/*Author Scott*/



public class NPCDrops {

	

	public static List<NPCD> npcd = new ArrayList<NPCD>();

	

	private NPCDrops() {

	

	}

	

	public static List<RealDrop> roll(int id, boolean ring) {

		List<RealDrop> list = new ArrayList<RealDrop>();

		for(NPCD n : npcd) {

			if(n.npcId == id) {

				for(Drop d : n.drops) {

					if(ring) {

						double rolled = Misc.randomDouble(0, 100);

						if(d.percentage	<= 5)

							rolled /= 1.5;

						if(d.percentage > 5 && d.percentage <= 10)

							rolled /= 1.75;

						if(d.percentage > 10)

							rolled /= 2;

						if(d.itemId == 995)

							d.amount *= 1.5;

						if(rolled <= d.percentage) {

							list.add(new RealDrop(d.itemId, d.amount));

						}

					} else {

						if(Misc.randomDouble(0, 100) <= d.percentage) {

							list.add(new RealDrop(d.itemId, d.amount));

						}

					}

				}

				return list;

			}

		}

		return null;

	}

	

	public static void loadDrops() {

		try {

			npcd = new ArrayList<NPCD>();

			Scanner scan = new Scanner(new File("./Data/cfg/NPCDrops.RBO"));

			scan.nextLine(); //[Format ... ] useless

			String s = scan.nextLine();

			while(!s.equalsIgnoreCase("[EndOfFile]")) {

				if(s.startsWith("#")) {

					List<rop> list = new ArrayList<rop>();

					int npcId = Integer.parseInt(s.substring(1, s.indexOf(" -")));

					s = scan.nextLine();

					while(!s.startsWith("#") && !s.equalsIgnoreCase("[EndOfFile]")) {

						list.add(readDrop(s));

						s = scan.nextLine();

					}

					npcd.add(new NPCD(npcId, list));

				}

			}

			scan.close();

		} catch (Exception e) {

			e.printStackTrace();

		}	

	}

	

	public static Drop readDrop(String s) {

		String[] args = s.split(":");

		return new Drop(Integer.parseInt(args[0]), Integer.parseInt(args[1]), Double.parseDouble(args[2]));

	}

}



class NPCD {



	public int npcId;

	public List<rop> drops;

	

	public NPCD(int npcId, List<rop> list) {

		this.npcId = npcId;

		drops = new ArrayList<rop>();

		for(Drop d : list) //avoiding weird referencing stuff

			drops.add(d);

	}

}



class Drop {

	

	public int itemId;

	public int amount;

	public double percentage;

	

	public Drop(int a, int b, double c) { //bein' lazy

		this.itemId = a;

		this.amount = b;

		this.percentage = c;

	}

	

}

NPCHandler.java (replace dropItem() and add the bone thing)

    	public void dropItems(int i) {

		long start = System.currentTimeMillis();

		Client c = (Client)Server.playerHandler.players[npcs[i].killedBy];

		if(c != null) {

			boolean wealthring = false;

			if(c.playerEquipment[c.playerRing] == 2572)

				wealthring = true;

			List<RealDrop> drops = NPCDrops.roll(npcs[i].npcType, wealthring);

			if(drops != null) {

				for(RealDrop r : drops) {

					Server.itemHandler.createGroundItem(c, r.item, npcs[i].absX, npcs[i].absY, r.amount, c.playerId);

				}

			}

		}

		System.out.println("Took: " + (System.currentTimeMillis() - start));

	}

If you want to be able to reload without restarting server, add this in Commands.java

    import server.model.npcs.NPCDrops;

and

    			if (playerCommand.startsWith("reloaddrops")) {

				NPCDrops.loadDrops();

				c.sendMessage("Drops reloaded.");

			}

In Server.java, add this in main()

    import server.model.npcs.NPCDrops;

    		NPCDrops.loadDrops();

In Data\cfg (where item.cfg and stuff are), add this file and name it NPCDrops.RBO (I had a big ego that day and named the file after myself. Feel free to rename lol.) Just because it's .RBO doesn't mean you can't modify it; just open it in Notepad or Notepad++ or Eclipse or something.

    [Format: [item id]:[amount]:[percentage (out of 100)]. 995:1:100 means drop 1 gold with a 100% chance.]

#1 - Man

995:100:100

#7 - Farmer

995:10000:100

14484:1:20

#8 - Thief

995:5000:50

15486:1:12

[EndOfFile]

How to use:
#[id] indicates a new NPC ID. If you want to add drops to an NPC, just do something like #1 - Man. You need to have at least "#1 -", you don't need to name the NPC.
After that, on separate lines, do this format: itemid:amountToDrop:chanceOfDrop (out of 100)

995:5000:50 - Drops 5000 gold with a 50% chance
14484:1:20 - Drops 1 Dragon Claws with a 20% chance
14484:10:.001 - Drops 10 Dragon Claws with a 0.001% chance