Love, War, and Tenchi Muyo!: This site has plenty of information on Tenchi, including short descriptions of the differences between the universes of Tenchi Muyo! It also features a growing songs, product description, and detailed character profiles sections - just to name a few!
GenSao's Tenchi Muyo Home Page: This is a great comprehensive site with a complete listing of episode summaries and tons of downloads - midis, movies, wavs, screen savers, and games! Also has a chart showing the breakdown of the separate Tenchi Universes.
Archen's Guide to Tenchi Muyo: If you're new to Tenchi, this is a nicely designed starting point. Has some good character information on the major figures of Tenchi Muyo, as well as a very pleasing collection of images.
Tenchi Muyo! Ryoohki!: Very simple page, but all the basic information you need to learn more about Tenchi Muyo.
