https://github.com/fn2006/PollyMC - Fork of prism launcher to allow offline play without an account.

UltimMC is good. It's a fork of MultiMC that allows playing without account.

SKLauncher allows custom skins and capes

Mineshafter[dot]info is what I used to use back in my cracked minecraft days. It basically provides a modified launcher that patches itself and the Minecraft JAR to look for the Mineshafter login servers instead of Microsoft's. It lets you use singleplayer as well as multi-player on cracked servers that do not force online authentication.

The site is still up and paid for, which means someone still gives a shit about it, so I think it may still be functional. I haven't used it in literally ten years though, so I doubt it supports the latest Minecraft versions though, but you can try it. Oh God. Ten years. I'm old...

Since you are looking to play single player you might also want to check out Minetest with the Mineclone2 addon installed. It is not identical, but it is an amazing substitute and free.

PollyMC. It's PrismLauncher but without DRM.

HMCL on github