https://www.sidefx.com/tutorials/ - Sidefx’s own learning resource platform with countless amazing content
https://entagma.com/ - Super high quality houdini tutorials, for beginners and advanced folks alike
https://www.tokeru.com/ - CGwiki - tons of amazing info
https://www.youtube.com/user/rohandalvi - Amazing tutorials and a has a great beginner series (that is where I started)
https://www.indie-pixel.com/ - Great courses lots of which are more focused on game dev. His Youtube is great too!
https://www.youtube.com/c/JunichiroHorikawa - Pretty advanced stuff but so much knowledge there. Cool thing is that you can download many of the project files and more on his Github.
https://www.houdini.school/ - Ever growing amazing set of courses for all skill levels.
https://www.youtube.com/c/Rebelway - Premium courses but also lots of free tutorials.
https://github.com/qLab/qLib - Tons of super useful nodes and goodies making day to day things a lot easier. - Free
https://github.com/Aeoll/Aelib - Inspired by qlib with more goodies. - Free
https://www.motionoperators.com/ - MOPS - A free toolset primarily focused on Motion Graphics in Houdini but also useful for a lot more. A must have! - Free (More advanced tools available to purchase (Mops+)
https://origamidigital.com/ - OD tools: A crazy amount of workflow improvements for Houdini including drag and drop functionality, automatic materials from texture sets, an asset library and much more. I don’t know how I could Houdini without OD-tools - $99.95 as of July 15, 2022
https://alexeyvanzhula.gumroad.com/ - Houdini Modeler - A great toolset to do direct modeling directly in Houdini.
https://www.cgcircuit.com/asset/hos-fast-relax-deformer - Vellum like but almost instant cloth deformer