I consider this essential if you intend to be webbrowsing: GitHub - raffaeleflorio/qubes-url-redirector: This browser extension, designed for Qubes OS, blocks and/or redirects non whitelisted URLs to another qube of your choice.
This could go under security: GitHub - unman/qubes-print
This could go in howto guides: WORKING: Guide for creating a logging qube (I.E. sys-log) - WIP
Probably several other things in the “guides” category could be put in the howto guides. The guides category is here: Guides - Qubes OS Forum