Kindle 3 Jailbreak and Hack will allow you to add and change some extra fonts, USB Networking and some screen saver hacks.
Following is the list of programs used in this hack.
clarknova for the first 2.5.x jailbreak & screensavers hacks
porkupan for one of the first bind mount based fonts hack
kukyakya for the usb watchdog & update-safe tweaks
jyavenard for the packager
tedsan & blogkindle for their fonts hack
You can read more about this one mobileRead and try it on your own risk. Personally I will not jailbreak unless I really need to test and use a test environment where I can afford to loose.