I purchased an iPhone 4. What can I do with my old iPhone?
Via the internet

You have four options: use it without the phone functions (making it an iPod touch), pass it on, sell it, or keep it as a spare. 

If you choose to pass it on, you should wipe all the data from it. The person receiving it can then set it up as a new phone with the phone’s carrier. Apple provides instructions for doing just that (www.macworld.com/6301).

Instead, you could sell it to companies that purchase old iPhones, but first try selling through sites like eBay or Craigslist – you may get a better price.

If you’d like to use it as a spare phone, you could unlock it (therefore voiding your warranty) and use it with a different carrier. This lets you employ a local (and less-expensive) SIM card when travelling overseas. The iPhone Dev-Team’s site (www.macworld.com/6298) offers instructions on unlocking an iPhone.