I held onto my 450D for a while after getting a 7D as a bit of spare and thought I'd give my partner a try on it to see if she'd become more interested in photography. That didn't really happen so I thought I'd sell it while it still had a decent resale value, it was in good condition but I was surprised how much I got on E-bay, only about $100 less than a new grey market 550D that would have come with a 12-month reseller warranty versus none at all. I put that towards a 5D I just purchased.
My previous cameras were digital P&S. The first Kodak I gave to my dad, it was in 'very used' condition but thought he might like it because he had the same model and might have found a use for the accessories if nothing else. The other was a FinePix that died when the USB connector snapped off. I removed the broken connector to see if I could replace it but something must have shorted and it was entirely dead.
That last one must have been sitting on my desk about a month without batteries and I pulled it apart to scrounge some parts that might be useful (I work with electronics). Out of habit I shorted the flash capacitor before removing it and was amazed I got quite a big spark after that amount of time. It probably wouldn't be fatal but at 200V or more (it was rated for 320V) at the least it would cause a lot of pain and maybe pull your arm out of join in the reflex action. Anyway just a warning, it was only a P&S so I imagine something like a 580EX would have have a cap that would store more energy.
Other than being careful of that I'd recommend pulling everything apart that's broken or too old to be useful to see how it works