Tested on firmware 3.2.1 and Kindle Keyboard. The fonts should work fine on any jailbroken Kindle, but I have only tested them on the Kindle 3 Wi-Fi.
I tried to get the Georgia2 fonts from a previous jailbreak to work but I had all sorts of problems with the Kindle locking up and not being able to enter the settings screen, so I decided to edit the font myself. In the process I edited several other fonts. The archive contains bolder, more legible versions of them:
|ITC New Baskerville||Serif||Screenshot|
|Times New Roman||Serif||Screenshot|
.ttffiles from the folder of the font you'd like (the files are named: Serif_Regular.ttf, Serif_Italic.ttf, Serif_Bold.ttf, Serif_BoldItalic.ttf)
K:\is your Kindle)
For help please consult the Mobile Read thread.