This mod adds a lot to the vanilla loot system, including new loot conditions, functions, and loot types.
Head over to the Loot Overhaul Wiki for a full list of features and some examples of how to use them.
Just a warning, this mod is complex! If you have questions, ask them below, and hopefully myself or someone else will be able to help, but you may have to figure it out on your own. The wiki is your friend. =)
Editing block drops is now as easy as editing loot tables!
You can replace (or add to with _extra, see below) any block drops using loot tables.
To replace block drops, use the loot table with the name :blocks/. So you can override vanilla grass (minecraft:grass) drops with the loot table minecraft:blocks/grass. For a modded block, lets say with the id biomesoplenty:mud, it would be biomesoplenty:blocks/mud.
Block loot table drops can be disabled in the config file (in case it causes issues).
Additions to loot tables (without replacing them)
Sometimes when making a custom map, or modpack, you don't want to replace a whole loot table. You just want a way to add a couple items to it. With this mod, you can do that too.
To add to a loot table, simply create another loot table with the same name as the one you want to add to, with _extra at the end (so adding extras to entities/zombie would be in entities/zombie_extra).
This feature can be disabled in the config file, or you can blacklist certain loot tables from trying to pull in extras, in case you have a mod that happens to have a loot table ending in _extra.
Additions to every entity death or block drop table
The two loot tables minecraft:entities/all and minecraft:blocks/all allow you to add a drop to every entity death loot or block drop, respectively. This would probably be most useful combined with loot conditions, for extremely rare drops and the like.
Both of those can be disabled in the config file.
New Loot Types
Well, kind of. The loot system currently only supports items, so there are 6 items which will have a special effect when they drop: spawning mobs, placing blocks, creating structures, filling an area, running commands, or applying a potion effect. See the wiki for examples of how to use them.
This mod adds in a fair number of new loot conditions, ranging from biome to moon phase to those entity/global variables mentioned above. See the wiki for more detailed descriptions and some examples.
There are also several new loot functions, with more detail on the wiki.