Each WikiWikiClone seems to use its own markup language. There was another markup language, Setext, designed to work well with plain text. There was even a PerlLanguage filter for it, so it was ready for use in wikis. It seemed well thought out.
Update, 2003: Work on setext seems to have stopped in the mid-1990s, and the format appears to be moribund. Most links to documents about it are broken.