The No Nether Mod

This mod is intended for players who have disabled the Nether for whatever reason (e.g. modern or prison-themed server; small server with Nether disabled to reduce lag) but still want a way to obtain nether items in ways besides using simple recipes that other mods out there do (although there are recipes in this mod too).

The first thing you will want to look for is blaze bushes (which naturally spawn in the world).  They in turn can be smelted down to blaze powder, which is the primary item for most of the recipes in this mod.

//i.imgur.com/N4RfXSB.png

Blaze powder can then be used to craft netherrack, netherwart, soulsand, ghast tears and wither skeleton heads.

//i.imgur.com/ykUjSTL.png

Nether Quartz can be made by smelting Quartz Ore, which spawns in the overworld underground.  

//i.imgur.com/yKncQ7S.png

Glowstone is also craftable

//i.imgur.com/8jAFzEw.png

To craft wither skeleton heads, you need a regular skeleton head.  Other than occasionally dropping from vanilla skeletons, you can also obtain skeleton heads by killing a new mob called Collectors.  Collectors can be "summoned" by crafting and placing down the collector's item (looks like a fancy golden egg).  Collectors also have a rare drop of ghast tears.

//i.imgur.com/Gr1yq8y.png

Since we all love to fight blazes, this mod also has an overworld equivalent of blazes called Pyros.  Pyros can be summoned by placing down a Pyro Beacon.  Pyros have similiar AI as a blaze and will drop blaze rods and magma cream as a rare drop.  Be ready for "unpleasantness" when you break the beacon!

//i.imgur.com/1jqXwyo.png

By the way, there is a rather annoying but ultimately harmless bug when you place down a pyro beacon or collector's item - it will also spawn a second "ghost" image of the mob.  You will see it but it has no AI and thus will not actually harm you.  Logging out and logging back in is a quick way to get rid of this "ghost" mob.