I've been working on a compiling a list of free/open source Houdini assets, tools, plugins, blogs, etc. You can find it here. Feel free to share anything I might have missed here and I'll add it.

https://github.com/wyhinton/AwesomeHoudini/blob/master/README.md [github.com]

UPDATE: Have reorganized the list and added a bunch of different open source packages. Some notable new finds include ZENV (https://github.com/CorvaeOboro/zenv) and laidlawFx (https://github.com/LaidlawFX/LaidlawFX)!

Don't forget to check out the blogs and personal sites sections, they often host some great HDAs!

https://github.com/wyhinton/AwesomeHoudini [github.com]