VEX, Python, OpenCL, and scripting in Houdini.

1 Minute VEX. Short lessons teaching Houdini VEX one small snippet at a time.
Houdini OpenCL (MysteryPancake). Guide to writing OpenCL kernels in Houdini.
HouPy Wiki. Houdini Python wiki. Packages, shelf scripts, menu customization, node graph hooks.
See also: Learning, Community & Industry → 3D Communities & Forums
Learning VEX via Bees and Bombs. OdForce thread recreating Bees and Bombs animations in VEX.
Mantra LPE Cookbook (alexwheezy). Light Path Expression recipes for Mantra rendering.
OpenCL in Houdini. OpenCL notes for Houdini.
Python Snippets for Houdini. Wiki page of Python snippets for scripting Houdini, part of kiryha's Houdini learning repo.
Python Startup Scripts. Houdini Tricks guide to writing Python startup scripts that run when Houdini launches.
VEX Manager Paid. VEX snippet/code manager HDA by Shir Manor.
See also: Modeling, Sculpting & Texturing → Houdini: Plugins & Tools
VEX Wrangle Cheat Sheet. Quick VEX reference by Mr Kunz.
vex_tutorial (jtomori). Practical introduction to VEX in Houdini with examples.
VFXBrain VEX. Blog with VEX tutorials and code snippets for Houdini.