TUAW has a great article on donating and discarding an old Mac: How to safely dispose of an old Mac. I didn't know Apple would accept an old machine and/or monitor upon purchase of a new machine, so one strategy is to keep the junk in the attic until you buy a new one.
Which reminds me of a post I never finished. Palm may be a vampire leaching off pale customers and forgotten innovation, but they are trying to look less bad by indirect means:
... Palm's recycling program is completely free -- that includes the cost of shipping -- and takes cellphones and handhelds in any condition, from any brand and also any old accessories that go with it.
All consumers have to do is download a pre-paid mailing label from Palm's web site, or pick one up at a Palm retail store. This service is apparently only available in the United States....
The Brighthand article has links to directions for erasing PDA data.
BlackBerry has yet another option. They'll not only recycle your old phone/smartphone, they'll give you "fair market value" credit towards your new BlackBerry:
Take your existing wireless device and turn it into a brand new, slim and stylish BlackBerry® smartphone. The BlackBerry Trade Up Program gives you the fair market value of your existing wireless device to use towards the purchase of the BlackBerry smartphone of your choice.
If your cell phone actually works many local groups will take them for 911 calling (women's shelters, etc).
On the other hand, nobody wants pre-XP computer donations. Don't bother, that's a purely recycling project. A good CRT can still find a home, especially for people with visual impairments who suffer from the fixed-pitch LCD problem. I think a group that works with the visually impaired might be interested in those, or look for an elderly computer user.
Ok, so I've done my insignificant part for the environment. Now if I can only figure out a way to get rid of all those plastic baggies ...