Spent fire extinguishers

So what does one do with spent fire extinguishers?

I've got a small extinguisher for the kitchen, and a medium sized one for the garage that are both sitting at empty. The garage one just seems to have depressurized over time. The kitchen one I had to use to put out a small fire in the oven that started up from stuff spilling over.

The kitchen one says "Discard/Replace extinguisher if pointer reads empty" on the pressure gauge. The label also says "Must be discarded" in tiny bold letters (it's a small label), so I take this to mean that this particular extinguisher can't be recharged. So what's the proper way to dispose of one of these? It's probably still got chemicals in it, so I'm hesitant to just toss it in the trash. Plus it's made of metal, so it could probably be recycled.

The pressure gauge needle on the garage one is sitting in the Recharge zone, and says "MUST BE RECHARGED" in slightly larger bold letters, so I assume this particular one can be recharged. I figure I can probably find a place in the phone book that does fire extinguisher service.

I should make a trip to the fire station down the road. I bet they can tell me what to do.