myts-8 is one of the better Kindle Keyboard terminals. Prerequisites are Jailbreak + some way of invoking the scripts e.g. Launchpad (recommended), KUAL or Kite. You can then use the built in VI editor which is esoteric and somewhat limited but entirely adequate for plain text. Heck, I've even used it to review and edit large chunks of C code (not even for a kindle) while on the bus!