If you jailbreak, you have the option to change your iPhone's default boot logo. You've always been able to do so manually through SSH, but recent jailbreak tools allow you to install custom logos in a much simpler way through Cydia.
If you're unfamiliar with the term "boot logo," it simple refers to the icon that displays on your iPhone's screen during a boot. Through the miracle of jailbreaking, it's rather easy to replace that Apple icon with all sorts of interesting things to look at.
Here are 10 of the best custom boot logos in Cydia...