VEX, Python, OpenCL, and scripting in Houdini.

1 Minute VEX. Bite-sized VEX lessons.
Houdini OpenCL (MysteryPancake). Guide to writing OpenCL kernels in Houdini.
Houdini Tech Blog (discretization). Technical blog on Houdini math and geometry processing.
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. Python snippets wiki.
Python Startup Scripts. Python startup scripts guide.
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. VEX tutorials blog.