Tax Free Levels

Reaching high levels in Minecraft is hard, because each level requires more XP than the last.
Since Anvils and Enchantment Tables work off of levels, not off of collected XP, that means the higher your level, the higher the actual cost in terms of XP.

Tax Free Levels flattens the curve of the ever-increasing anvil and enchantment costs, meaning you'll always pay the same amount of XP, regardless of how high your level is.
It by default also removes the "Too Expensive!" anvil limit and makes it so that renaming an item always only costs 1 level.

More precisely:
Instead of paying X levels in an Anvil, you pay the amount of XP needed to get from level 0 to X.
If your Enchanting Table has a level requirement of 30, instead of paying 3 levels, you pay the XP needed to get from level 27 to 30.

Note: For version 1.3 the XP cost is always calculated as the amount of XP needed to get from level 30-X to 30, having no effect on costs below player level 30. This behavior can be restored in 1.4 by setting the "Level Base" to 30. Version 1.3 also does not remove the anvil limit.

for Minecraft 1.16 and above
requires Cloth Config. Fabric also requires Fabric API. (Both since 1.4)
required on the server and client
(it does work server-only, but there's issues like the "Too Expensive!" message popping up and the level cost not showing correctly when renaming items, but those are purely visual.)
compatible with Draylar1's Reroll
compatible with Enchanting Overhauled
compatible with Enchanting Infuser
compatible with Charm / Charm Reforged's Grindstone enchantment extraction
compatible with Spectrum's Bedrock Anvil
compatible with Waystones (older (Neo)Forge versions, newer ones use an XP cost by default. Fabric tip: Wraith Waystones can set fixed XP cost)
compatible with Apotheosis / Zenith (7.2.0+ / 1.0.0+ cost calculation will be replaced, note that the shown XP cost isn't always accurate)
compatible with Tetra's Workbench (1.18 - 1.20.1)
compatible with Dis-Enchanting Table (1.21.1 Fabric/NeoForge, 1.20.1 Fabric/Forge)
compatible with DisenchantingForge (1.18 - 1.21.1) and Fabric (1.18 - 1.20.1)
compatible with Twilight Forest's Uncrafting Table (Forge 1.20.1)
compatible with Alex's Mobs' Transmutation Table (Forge 1.20.1)
compatible with Backpacked (1.20.1+)
incompatible with Balanced Enchanting (the mod does effectively the same thing as this mod, minus all the compatibility stuff, choose one or the other)

Comparison

Say you've just slain the Ender Dragon for the first time and reached level 68.
In Vanilla, you can level 3 enchant a total of 13 items.
With Tax Free Levels, this changes to 35 items, your Ender Dragon kill actually counts for something!

Say again, you are level 68 and want to apply another enchantment on your pick which has a very high repair cost and it'd cost 35 levels.
In Vanilla, you pay 10220 XP (the XP needed to get from level 33 to 68).
With Tax Free Levels, you pay 2045 XP (the XP needed to get from level 0 to 35).
You'd normally lose 8000 of your 12000 XP dragon reward here and there!

No Small Tax

One small tidbit about vanilla Minecraft is that paying levels doesn't actually touch the percental experience progress.
Since higher levels are worth more XP, this means you actually not only pay levels but also a small amount of XP.
Tax Free Levels gets rid of this tax as well, even at lower player levels.

Attribution

This is meant as a partial replacement for user11681's limitless, specifically the "level normalization". No code was taken, only the idea.