There are two OAVs and one TV series spin-offs of Tenchi Muyo! They are Magical Girl Pretty Sammy (Pretty Sammy for short) and Galaxy Police Mihoshi's Space Adventure (Mihoshi's Space Adventure). Both of these are OAVs, Pretty Sammy has its own TV series as well.
Mihoshi's Space Adventure is a special one episode series completely seperate from the OAVs or TV series. Mihoshi tells the story of how her and her partner Kiyone brought down the worst space pirate in the Universe. To tell her story, she uses the Tenchi characters to portray the characters in her story. Her story goes from unbelieveable to impossible as they travel to space to save "Investigator Tenchi" and a mysterious, magical girl named Pretty Sammy saves them all from a mad scientist's "Galaxy Destroyer."
We first saw Pretty Sammy in the Mihoshi special, she soon got her own short OAV series. In the series, Tenchi Masaki becomes Tenchi Kawai, Sasami's older brother. Ryoko and Ayeka are high school students who are always fighting over Tenchi (as usual). Washu is the science professor at the high school. Mihoshi and Kiyone are college students who work in Sasami's mother's CD shop, CD Vision.
Sasami was chosen by Tsunami, a candidate for the crown of Juraihelm, to become her champion and transform into the Magical Girl Pretty Sammy. She must fight for love and justice in order to earn Tsunami points towards winning the crown. Tsunami's rival and other candidate, Ramia, uses Sasami's friend, Misao, as her champion. Misao becomes Pixy Misa and creates mischeif and chaos of a whole new breed. Friends must fight against each to see if good really can triumph over evil, no matter what the cost!
Magical Project S
Pretty Sammy goes a step farther and comes out with her very own TV series. In the US it is known as Magical Project S. The Tenchi characters return as they were introduced in the Pretty Sammy OAV. Both Pretty Sammy and Pixy Misa (as is Misao) are back. Magical Project S succeeds in "lampooning" every magical girl series under the sun from SailorMoon and beyond, as Pretty Sammy fights Pixy Misa and her 'love love' monsters in the name of love and justice.