What Are Your Must-Have Extensions?

Hello, is there an extension that you consider a must in your community? If so, what is it? I want to start with myself.

The Mobile Tab extension definitely makes the forum especially mobile users feel more comfortable, the extension link: https://discuss.flarum.org/d/28216-mobile-tab

Flarum Categories... it's the first thing you experience when you hit PianoTell.

Cross references (n-peugnet). I hope new mentions extension will replace this.

tolgaaaltas I haven't experienced it before but it looks like a good thing.