5X - The Ultimate Balance Mod for Master of Orion

Installation instructions (Windows): To use 5X - The Ultimate Balance Mod, download the latest version, and the unzip the file and move the 5X - The Ultimate Balance Mod folder to Documents\Master of Orion\Mods. It should then show up in game in the Mods menu as a "Local mod." All other normal MoO mods are installed in the same way.
Highly recommended for use with or without 5X is WhatIsSol's Unofficial Code Patch. Download the optional 5X UCP Compatibility mod (5XCPC) from the 5X page and make sure the order of the mods in game is 5X, UCP, then 5XCPC.

I'm not sure how to use a mod on the Mac or Linux version, but I think there's a similar Mods folder somewhere you're supposed to move the folder to.

5X is a complete overhaul of the gameplay experience. Here's a summary of some of what this mod does in no particular order. This is not a complete list:
Improved AI personality and ship designs. Multiple personalities available for the AI so they can act differently each game.
More anomalies with nicer rewards
Rebalanced custom race traits and race designs. All races now cost the same amount of points (20) and can be reproduced on the custom race screen. Revealed all hidden custom race perks and added new ones. Point budget increased to 20 to allow for finer granularity of trait balance.
Jump gates reduce travel time by 25% instead of 50%
Scrapping ships and structures gives more BC, as does trade goods.
Rebalance of command point economy
New ship hulls
Added tooltips for various structures/ship stats/weapon stats
Rebalanced pirates
Rebalanced spy missions, techs, training costs, and security bonuses. Spy max level increased to 10
Research and trade treaties are cheaper but provide less benefit, and they last 40 turns (need UCP for research treaties to last 40 turns)
Increased max number of ground combat units per colony (for defense), but you start with less and they train slower
Repeated use of terraforming doesn't cost as much (requires UCP)
Can build ships in stack sizes up to 100 (vanilla is capped at 5)
Biome degradation from pollution happens 1 turn after hitting biome cap.
Upgrading ships is a bit cheaper
Armor resilience can now reduce damage up to 50% instead of 25% (requires UCP)
Doubled tactical battlefield size
Complete rebalance of leaders
New game setup options:
Galaxy types:
Irregular galaxies have random distributions of stars with no limit on star lane length.
Vine galaxies have one connected "path" through the galaxy with branches off of the main path. Very heavy on choke points but there also more wormholes in this type.
Web galaxies: Every star in a cluster is connected to every other star in every other cluster. Each system has two entry points, one stable (intra-cluster travel) and one unstable (inter-cluster travel)
Ring/annulus galaxy: Shaped like a ring but has some unstable star lanes breaking it up into sections
Double galaxy: two main sections of stars connected by a couple of paths between
Large triple galaxy: three main sections of stars
Huge antispiral: Huge spiral galaxy but everyone starts in the middle
Unstable: Has unstable (red) warp lanes throughout the galaxy, but you can still reach every star without them by taking a longer route.
Galaxy sizes: Added several size options to circle, spiral, and cluster
Galaxy ages: Very young and very old added. Also added lush/very lush (good minerals and good biomes) and spares/very sparse (bad minerals/bad biomes)
Planet densities: very low and very high
Starting ages:
Pre warp tech tree expanded to 5 nodes
Hyper advanced: Start at the end of the vanilla tech tree (where the 5X endgame tree starts)
Rush: Start a bit beyond post warp with 9 colony ships
Advanced rush: Start somewhere between advanced and hyper advanced with 9 advanced colony ships
Lopsided: Start with all the techs leading to teaching methods (astro university) but none of the other techs
Multistate insulator: All of the techs leading up to multistate insulator but nothing else
Microlite construction: All of the techs leading up to microlite construction but nothing else
Difficulty: Added "Impossible" difficulty, where the AI gets a +100% bonus to food/production/population/command points/money (extreme has +50%)
Added more timeline save frequency options. Choose to auto-save less often to decrease save file size and reduce turn processing time.
Galaxy Generation:
Rebalanced the number of stars/wormholes/minor civs/monsters in each galaxy type
Added a new star type, Black Hole, which mostly has no planets or just asteroids, but planets that are found are typically terrible biomes but rich in minerals. Very small chance to find an uber biome around a black hole. (Coming in v4.3)
Very small chance to find an uber biome around a brown star. (Coming in v4.3)
Outer orbits have a small chance to spawn multiple objects in the same orbit. That means it's possible, though rare, to find a system with 6 planets.
Wormholes have a chance to have asteroid belts in them (Coming in v4.3)
Added a couple new planetary resources. There are now techs that can be built only on planets with certain resources that add to the resource benefits.
Biomes:
Rebalanced biome stats a bit. Deserts are now slightly better than tundras.
Added new uber biomes for Alkari, Bulrathi, Darlok, Meklar, and Trilarian. '
Races with an uber biome now get a unique building (different bonuses for each biome type) that can be built on their homeworld only, and an upgrade to this building can be built on any of their uber biome planets.
Orion now has its own unique biome type (uber gaia) with more food output and pollution tolerance than a normal gaia. Defeating the Guardian will unlock a unique building that can only be built on gaias and Orion.
Production bonus perks now come with a building tech that reduces pollution. If you are using UCP, this is replaced with the percent pollution reduction perk.
Rebalanced lithovore growth rates
Rebalanced a variety of ship hull stats. Increased available special/weapon slots of each hull size by 1 among other things.
Added new planetary defenses and strengthened defenses in general. Military outposts are stronger and have to be researched.
Rebalanced all weapon stats and ship modules. Cannons are good vs shields, beams good vs armor in general. Fast missiles now increase the range of missiles as well. You start with better beam attack, so missiles aren't as OP early game.
Rebalanced all ground combat techs.
Tech Tree:
Rearranged several techs. For example, you now start with troop transports. Added a lot more choice nodes to the tech tree.
Added several new technologies to the tech tree. Too many to go into.
The tech tree has been extended by several nodes to include some hyper advanced technologies as well as several tiers of miniaturization techs so your endgame research isn't for nothing.
Rebalanced tech tree research point costs. This should slow down research progression overall.
Rebalanced Antarans (their ships are stronger but they should send smaller fleets in general. Their fleet size is still based on your fleet size though, so as you ramp up so will they).
Improved minor civ rewards.
GNN gives "state of the galaxy" reports further into the game (previously they would stop around turn 300).
Rebalanced the pollution system (some features require UCP).
Rebalanced maintenance cost for all buildings.
Rebalanced/changed several structure bonuses.
Orion guardian in stronger
New features and tweaks with every update. Check the change notes for the latest changes.

Feedback and suggestions are welcome. Please realize that the modding system in MoO is very limited. I can't change the user interface, import custom assets, fix bugs in the game that are unrelated to the mod, or in general add anything to the game that is not already coded into the base game.

Languages available: English, Russian, German, Spanish (Spain), Polish. French is available for version 2.3.1 and earlier.

Donations are welcome and can be made through the Nexus Mods 5X page.

I hope you enjoy the mod!

Credits:
Author: Spud Dastardly
Testing and Assistance: Exoclyps and Vaaish
German Translation: Stefan_r and Sensei
Russian Translation: Reriser
French Translation: Laegad, Random
Spanish Translation: Mursilis
Polish Translation: Szmind
DSMP Assets: Morpheus
Special thanks: WhatIsSol and Javierota