First, a working iMac is worth way more than you could get for the parts. You can still sell even an older imac for a pretty good price. If you would prefer a non-imac, sell the thing and build a new machine. If it's in good condition, it should be worth at least $400.
If you are intent on scrapping the thing, you can reuse the CPU: it's probably an i5 2400s. You will need an LGA 1155 (sandy bridge) motherboard.
You can reuse the RAM-iMacs us SODIMMs, so you could use the RAM in a laptop or small form factor PC that uses SODIMMs.
You can reuse the hard drive-it's just a standard disk.
You could remove the GPU, but it would only be useable in another iMac.