I just replaced the 2 1GB DIMMs in my MacBook Pro with 2 x 2GB DIMMs. What can I do with the old memory?
If you don't think your gonna use it again, why not recycle it? Or you could use the sticks as a really geeky bookmark.
You can sell it, or - as tj111 noted - recycle it. Note: in EU countries computer memory is considered dangerous waste, and by law should be properly disposed by appropriate recycling company. Ask your local computer store, they will help you.