How do you get rid of old paint, cleaning solutions, etc?

I've got tons of paint, automobile cleaning stuff, garden poisons, etc that haven't been touched in decades and I think it is time for it to go.
Normally, I'll just toss the randam can of two in the regular trash, but I don't think the garbage man will take a whole sack full.
What do I do with it?

How do you get rid of old paint, cleaning solutions, etc?