I have my main gaming rig and my newest laptop that I use regularly. My previous main rig I still have and used to mess with Linux on it. Now it's back to XP and I let my roommate use it to play WoW. I had an older Compaq that I used years ago in my bedroom when I lived with my parents. I gave that to my parents for my mom to use. I've given even older systems away to friends. An even older main rig of mine, I fried the mobo. I reused many of it's parts in the next build. However, I offered the parts (as well as some other spares I had lying around) to my church. Preacher gave me a motherboard, I built another system of it and now it sits in that Church's computer lab. If it still works, I don't throw it away.
I've had various laptops. One I had 3 years ago (15" HP, Sempron 2800), bought it used off eBay and the motherboard fried in a year of owning it so I took it to Hazardous Waste Collection but kept the CPU and RAM. The next one (15" Compaq, Celeron 440M) I bought used from a friend, I had it for 2 years, broke it's monitor and bought a new Compaq (15", Athlon X2 QL-62). Something was wonky with it and it couldn't do things up to my standards so I sold it to a regular customer at the restaurant I worked (was perfect for them). I actually replaced the busted monitor in my old laptop and kept using it. Then my dad bought me a new Toshiba (16", Core 2 Duo of some sort) but it overheated and died within 30 minutes of using it. Returned and swapped it for a new HP (17", Turion X2 RM-74) and gave my old Celeron laptop to my dad. He still uses it.
If it still works, I either find a use for it, or give it to someone who could use it. If something on it breaks, I remove what's usable, and take the useless bits to Hazardous Waste Collection.