See Optifine Alternatives for a few useful client side only mods!
Mods on this list are marked as outdated when they are two major Minecraft versions old - e.g. if 1.16 is the latest version, 1.14 and older mods are considered outdated.
Bugfixes
| AntiShulkerDupe | Modrinth | fixed by 1.19.1, outdated | 
| Chunk Saving Fix | CurseForge | outdated | 
| DFASCF | GitHub | fixed by 1.16.5, outdated | 
| Dimension Mutability | CurseForge | outdated | 
| Dimension Update Fixer | CurseForge | outdated | 
| Dispenser Portal Fix | Modrinth | fixed by 1.16.5, outdated | 
| MC-198678 Fix | GitHub | fixed by 1.16.3, outdated | 
Building
Chat
| Chat History | CurseForge | outdated | 
| Fabric Death Messages | Modrinth | outdated | 
| GraphiX | Modrinth | outdated | 
| simplechat. | CurseForge | outdated | 
| Text Coloriser Plugin | CurseForge | outdated | 
| Json Chat Mod | Modrinth | outdated, abandoned | 
Join Messages/MOTD
| CustomJoinMessagesFabric | CurseForge | outdated | 
| EasyMOTD | CurseForge | outdated | 
Chat Linking
Discord
| DatChat | CurseForge | outdated | 
| Discord Fabric Whitelister | CurseForge | outdated | 
| Discord-Fabric Link | GitHub | outdated | 
| Interweave | CurseForge | outdated | 
Matterbridge
| MatterLink | CurseForge | outdated | 
Twitch
Claims/World Protection
| BineClaims | Modrinth | outdated | 
| SpawnProtectionTweaks | Modrinth | outdated, abandoned | 
Commands
| Block Quantity Scanner | Modrinth | outdated | 
| Command Alias Creator | CurseForge | outdated | 
| Connected | GitHub | outdated | 
| Default Spawn Points | Modrinth | outdated | 
| EqualNBT | Modrinth | outdated | 
| Gestus | CurseForge | outdated | 
| Handy Structures | CurseForge | outdated | 
| MaskOfLoki's Fabric Tweaks | CurseForge | outdated | 
| Mod Info Command | Modrinth | outdated | 
| pride | CurseForge | outdated | 
| Rich Chat | GitHub | outdated | 
| SetPlayerData | CurseForge | outdated | 
| Slapmap (maxheyn fork) | GitHub | outdated, fork of Slapmap | 
| Slapmap | CurseForge GitHub | outdated | 
| Tagboard | CurseForge | outdated | 
| capit_alists' gameplay commands | CurseForge | outdated | 
| Walaryne's Fly Mod | CurseForge | outdated | 
| Who's pet is this? | CurseForge | outdated, abandoned | 
Teleportation/Homes
| FabricHomes | Modrinth CurseForge | succeeded by BlossomHomes, outdated | 
| FabricTPA | Modrinth CurseForge | succeeded by BlossomTPA, outdated | 
| FabricWarps | Modrinth CurseForge | succeeded by BlossomWarps, outdated | 
| FabriKommander | CurseForge | outdated | 
| Random Teleport | CurseForge | outdated | 
| CyanSetHome | Modrinth | outdated, abandoned | 
| SimpleHomes | Modrinth | outdated, abandoned | 
Cursed
| Bedrock Breaking | Modrinth | outdated | 
| PacketIgnore | Modrinth | outdated | 
| Volatile Entity Cramming | Modrinth | outdated | 
Economy/Shops
| GShop | Modrinth CurseForge | outdated | 
Gamemodes/Minigames
| BattleMod | CurseForge | outdated | 
| Chest Games | Modrinth | outdated | 
| Fabric Observer Gamemode | GitHub | outdated | 
| Manhunt | Modrinth CurseForge | outdated | 
| Minecraft Manhunt | Modrinth | outdated | 
| SoulSplit (ThirdLife) | CurseForge | outdated | 
Inventory
| GVault | Modrinth CurseForge | outdated | 
| keepinventory-command | CurseForge | outdated | 
| SimpleServerSorter | CurseForge | outdated | 
| Starting Notes | CurseForge | outdated | 
NPCs
Performance
| hopperOptimizations | GitHub | requires Lithium fork, outdated | 
| RandomPatches | Modrinth CurseForge | outdated | 
Affects behaviour
These mods make significant changes that may impact behaviour in-game, or reduce the work done (e.g. ticking, chunk loading) by the server. Technical servers may not want to use these.
| Custom Item Range Combination | Modrinth | outdated | 
| Overworld Two | CurseForge | outdated | 
Quests
Server Management
| Defibrillator | Modrinth | outdated | 
| ModUpdater | CurseForge | outdated | 
| PathWatcher | CurseForge | outdated | 
| ReloadAnnouncer | CurseForge | outdated | 
| Scarpet Os Utils | Modrinth | outdated | 
| server-restarter | GitHub | outdated | 
| TBPlugin | CurseForge | outdated | 
| Fabrilous Updater | CurseForge | outdated, abandoned | 
| Gamerulify | Modrinth | outdated, abandoned | 
Anticheat/Anti X-ray
| CheaterDeleter | GitHub | outdated | 
| Simple Anti X-ray (SAX) | Modrinth | outdated | 
Backups
| Auto Backup | CurseForge | outdated | 
| Minesave | CurseForge | outdated | 
Console/Logs
| Shut Up Console | CurseForge | outdated | 
| Translated Server Log | CurseForge | outdated | 
| Fabric Console | CurseForge | outdated, abandoned | 
Datapacks
| Data Loader | CurseForge | outdated | 
| Datapack Easy | Modrinth | outdated | 
Gamerules
Logging/auditing
| DeltaLogger | CurseForge | supersedes WatchTower and BlockLogger, outdated | 
Multiworld/Dimensions
Permissions/Roles
| Better Op | Modrinth | outdated | 
| TeamPerms | Modrinth CurseForge | outdated | 
Pregeneration
Profiling/Debugging
| Schwarz | Modrinth | outdated | 
Protocol tricks - Cross-version/edition compatibility, resource packs for client-required mods
Proxy forwarding (BungeeCord/Waterfall/Velocity)
| Trampoline | CurseForge | outdated | 
Resource packs
Server List modifications
| Random Server Icon | GitHub | outdated | 
Statistics/Analytics
Tab List modifications
Tweaks
Cauldrons
| Cauldron Concrete | Modrinth | outdated | 
Combat
| You can't damage me! | Modrinth | outdated | 
Crafting
| Glowing Potion | Modrinth | outdated | 
| Logical Recipes | CurseForge | outdated | 
| Rotten Flesh to Feather | Modrinth | outdated | 
Dispensers
Enchanting/Enchantments
| Disable Riptide | CurseForge | outdated | 
Fishing
| Old Fish Farm | CurseForge | outdated | 
Flight
| BeaconFlight | CurseForge | outdated | 
| Disable Elytra Flight | CurseForge | outdated | 
| Totally Balanced Creative Flying | CurseForge | outdated | 
Game Mechanics Tweaks
| Bed Stability | CurseForge | outdated | 
| Bedrock Blocks | CurseForge | outdated | 
| Block Physics | Modrinth | outdated | 
| Burninator | Modrinth | outdated | 
| Disable Portals | Modrinth | outdated | 
| Falling Block Tweaks | Modrinth | outdated | 
| Lid Space | CurseForge | outdated | 
| Make Every Biome Spawnable Lol | CurseForge | outdated | 
| MaskOfLoki's Fabric Tweaks | CurseForge | outdated | 
| Oak's Seed Drops Mod | Modrinth | outdated | 
| Piston Control | CurseForge | outdated, works best with client mod installed | 
| Silky Spawners | CurseForge | outdated | 
| Spawner Config | CurseForge | outdated | 
| Trample Stopper | CurseForge | outdated | 
Graves/Death
| keepinventory-command | CurseForge | outdated | 
| Ueaj's Graves | CurseForge | outdated | 
| VanillaDeathChest | Modrinth CurseForge | outdated | 
Minecarts
Mobs
| Creepers Burn | Modrinth | outdated | 
| Daylight Mobs | CurseForge | outdated | 
| Farmable Shulkers | Modrinth | outdated | 
| Less Annoying Piglins | Modrinth | outdated | 
| MobCapped | CurseForge | outdated | 
| Neutral Mobs | Modrinth | outdated | 
| No Ambient Mobs | CurseForge | outdated | 
| No Striders | CurseForge | outdated | 
| Stop Strider Suffering | GitHub | outdated, abandoned | 
| Unsaddle Pigs | GitHub | outdated, abandoned | 
New Game Mechanics
| Flora Doubling | CurseForge | outdated | 
| Imbued Torches | CurseForge | outdated | 
| Notchify | CurseForge | outdated | 
| Splash Concrete | CurseForge | outdated | 
| VanillaEndervators | CurseForge | outdated | 
Villagers
| Hungy Villager | Modrinth | outdated | 
Utility / Quality of Life
| I Am Very Smart | CurseForge | outdated | 
| Playtime Tracker | Modrinth | outdated | 
| SimpleHarvest | CurseForge | outdated | 
Cosmetic
Sleep (Voting/Acceleration/etc)
| Better Respawn | CurseForge | outdated | 
| BetterSleeping | CurseForge | outdated | 
| BetterSleepPlus | CurseForge | outdated | 
| Hypnos | Modrinth CurseForge | outdated | 
| Lucid | Modrinth | outdated | 
| SleepPunisher | CurseForge | outdated | 
| Wake Up | CurseForge | outdated | 
Tree/Leaf removal/decay
| Leaf Me Alone | CurseForge | outdated | 
Web Maps
| TinyMap | GitHub | outdated | 
Worldgen
| Smooth Bedrock (<=1.16.1) | CurseForge | outdated | 
| Climatic World Type | CurseForge | outdated | 
| No Lakes | CurseForge | outdated | 
| NoWaterCaves | CurseForge | outdated | 
| SecureSeed | CurseForge | outdated | 
| Simplex Terrain Generation | CurseForge | outdated | 
| Struct Plus | Modrinth | outdated | 
| Swamp Oak Redux | Modrinth | outdated | 
| Upstream | CurseForge | outdated |