That depends on your definition of rip-off. If you you mean parody than Spaceballs is without doubt on the top of the list.
However, if you mean movies that take the general premise and/or style and attempt to cash in on the Star Wars wave, there are a few notables. In no particular order, here are a few movies that attempted to make a few bucks by following and appealing to the Star Wars crowd. I include in this list the good ones, that actually are entertaining to watch and are worthy of mentioning.
There are a lot of really terrible rip-off films out there, but we'll leave those for another list.
Star Trek: The Motion Picture
While not a direct imitation, the success of the Star Wars brought the TV series back to attention of the producers and the film was rushed into production in order to ride the Star Wars wave.
While essentially a fantasy movie, it is surprising to see that the movie included laser fights, weird alien monsters and sword-fights that remind a lot of lightsaber fights.
Battle Beyond The Stars
Rip-off king Roger Corman produced several movies actually in the wake of Star Wars, most notably this one. It is essentially a space-version of The Magnificent Seven, which in turn is a western-version of Seven Samurai.
The Black Hole
Disney scrambled to find their own space opera adventure, and this one became it. As such it is not a bad movie and surprising dark and violent for a Disney flick. And the robot-sidekicks are more Disney than anything else.
This is without doubt the most obvious rip-off, as it takes several elements from Star Wars and revamps them, such as lightsabers for example and even the Death Star (although one may argue it's actually a Planet Killer). There is however the added bonus that it is an Italian movie, the lead is a hot chick and it's got David Hasselhoff in it. I repeat, The Hoff is in it.
A nude 63-year old Kirk Douglas and a scantily-clad Farrah Fawcett. And a robot. 'Nuff said.