Since 2008 modularsamples.com has been providing synth samples, construction kits and other sound design tools online. This content has been available for free with no usage restrictions. In other words public domain. Freed from commerical concerns the sounds are often abstract, of niche use or just downright weird. That said, I haven't forgotten those who like a little bread with their butter as the site feature quite a few leads, pads and even simple recordings of the basic waveforms. If you want literally 100s of variations on saw , square and triangles? I've got it covered.
In short, there's a lot here. If it's all too much to digest, or download. Consider buying a drive from my Etsy store. They are filled to the brim with everything I've ever made, and by purchasing one you are helping out this site. .