I have a small collectio, not really as an investment, but because I think foreign currency is really fascinating. I also have a really cool $1 bill that's backed by silver rather than gold, and is from the first series that ever displayed the "In God We Trust" motto on the dollar bill in 1957.

I also have various franks and deutschemarks from before the Euro, some of the big 5 pence coins from before England changed them to really small, some Chinese coins that are totally bizarre (one feels and looks like it's made of plastic but makes a metallic sound; and another with a hole in it from 1944 [before the cultural revolution]), and of course your requisite old American coins like a penny from 1944 and quarters from the 60s.