GURPS Space is one of the many competent supplements for Steve Jackson's Generic Universal Role Playing system.
Or, handy when you want to go haring around in spaceships and that sort of fun stuff. Mostly we used it as an adjunct to the other games we were playing at the time, but worth a look.
3 out of 5
The above link is the complete list of post from this blog, updated each week, if you want to browse like that rather than search.
This list is quite long, and has become longer than I ever thought so in the interests of loading speed it is now situated elsewhere.
Similarly I am working on a simple website to browse by author that combines Free SF Reader and Not Free SF Reader that is extremely bare bones now, and can be found at the second URL above.