Elements!
Theodore Gray's excellent site about elements and chemistry, makes excellent reading. He also has a really nifty book out showing dangerous and impressive demonstrations of chemistry. I bought a signed copy, it's a really fun read. Granted, about a third of the things he shows are things I've tried myself, and a handful more are things I may well try in the future. The rest are something I'm not about to attempt alone!

While admiring his excellent element collection, I started musing about what sort of an element collection I could assemble from things I already have around. There are a lot of judgement calls involved, but here's a first cut. I could make a case for things like chromium and vanadium, as I probably have some bits of stainless and other interesting steel alloys, but I can't point to them, so I'm not counting them. Similarly with several other elements (potassium, fluorine, bromine, niobium, manganese, and so on).