Just a list of mods that can be used both in singleplayer and mutliplayer but do not add content (items, blocks, and mobs) to the game. If you know of any good ones not on this list, feel free to link them in a reply.
So far I have found these mods:
Also, for those who don't know, ssp is singleplayer, and smp is multiplayer.
Collections of Mods
A collection of some great mods, some of which are compatible with smp.
Also a collection of mods, CJB's Modpack features infomation about your world(smp or ssp) like if slimes are in the chunk, total days passed, time of day, FPS, and more. It also will say the seed of your current world(could be useful on an smp server). This modpack also has a minimap with waypoints, mobs, palyers, and items: and an x-ray mod. There are other features, but most of the ones not mentioned are ssp only.
A few mods, one of which is a minimap: ZanMinimap. It is a minimap that works with the Nether, Aether, and maps caves.
Helpful Mods (Mods that make you Minecraft life easier)
This mod lets you toggle mining, jumping, crouching, running, with toggable controls. This mod also has better 3rd person, looking behind you, and more.
This mod lets you cpy and paste, go back to what you typed, get the things you just typed, and scrolling for chat history. Also, when you leave a server, the chat is still there when you come back, even when you go to another server! Also, if you are someone who types a lot at once, dont worry, this mod will make text turn red, and put it in the next thing you say.
Is your inventory a mess? Or do you like things organized. Whatever your answer to those two questions, you should get this mod. You can sort your inventory the way you like. When you use up a stack of anything, if you have more of it in your inventory, it'll replace the blocks that you just used up.
Adds mods that do things with HUD and a mod that has mining speed that is changable ingame, and extended reach.
Have any suggestions? Post them in the comment section below. Also, have something like a banner for one of the mods without a banner? Give it to me, and I'll add it.
Oh, and by the way. Check with the servers you play on what is and isn't allowed before installning client-side mods. It would suck to get banned for cheating and lose everything you built now, wouldn't it.
Two most essential independent client mods to me are minimap (Rei's minimap or Zan's minimap/Voxelmap) and Damage Indicators (these are unitframes for mobs essentially).
If you're not running a minimap mod, or love cave exploring but keep getting lost, I'd also try MagicYarn.
Recipe Book is super useful if you have 1001 mod and can't keep all the recipes straight.