My iPod Video. My iPhone 2G had finally fallen to bits, so I was needing some money to buy the iPhone 4 at launch. I sold my TI-83 calculator, my then Samsung feature phone, Leopard OS X installer, and then the iPod. It was barely over $200 after eBay taxes and shipping.
I only regret it now because I didn't know how much the app situation was going to change and I thought the iPhone was still going to do me well as a music player. I was racking up 3GB of photos every month, and could hardly take advantage of iMovie because of both my photos and music took up most of the space.
The iPod Touch I have now is great. But I'm a few albums short of maxing it out completely. If I still had the iPod Video, I could mod it with a new drive and battery and not deal with having to wait around to see if they make a 128GB version of the Touch, or do something with the Nano.