Material Connexion – Great database. There are paid options if you want to dig really deep. If you live close to one of their material libraries, it would be worth the subscription cost.
Materia – Great database and all free.
Transmaterial – Blog on various materials and how they’re used. Fun application of materials!
StylePark – Webpage with database for designers. There’s more than just materials info at this site.
OpenMaterials – ” OpenMaterials is a research group dedicated to open investigation and experimentation with DIY production methods and uses of materials.” Great idea but doesn’t seem to get updated all that often. Still, it’s a fun site to check out.
Syndaver – If you’re looking for materials that mimic human tissue behavior, this is the place. If you’re squeamish, it might freak you out. 🙂 Don’t say you weren’t warned. (You’re going there, aren’t you? 😉 )