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...