2010 (1984)
Not as notable as Kubrick's original, but a watchable sequel that is much more conventional than its predecessor. It's never had a decent release on DVD or Blu-Ray, though.
Beneath The Planet Of The Apes (1970)
If you want to take a look at one of the sequels to Planet of the Apes, you'll probably be best just seeing them all. Fans will tell you that either the second, third, or fourth movie in the series is the best sequel (with the third installment, Escape from the Planet of the Apes, being the most favored), but most will agree that the final film in the series, (Battle for the Planet of the Apes) is the worst.
Bride Of Frankenstein (1935)
It's terrific, though I'd consider it camp horror long before I thought about applying it with the sf label.
Capricorn One (1978)
The sf elements (the voyage to Mars) aren't the key to the story. It's much more of a conspiracy thriller in the vein of All The President's Men or Three Days of the Condor. It's also directed by Peter Hyams, who helmed 2010: The Year We Make Contact.
The Fly (1986)
It's better than the original, or either of its sequels. But not for the faint of heart--David Cronenberg doesn't shy away from gore.
Gattaca (1997)
It's one of the best sf movies released in the 1990s, and the Blu-Ray has a terrific transfer.
Invasion Of The Body Snatchers (1978)
Again, I can't recommend this film enough.
Marooned (1969)
I haven't seen it, but it's mentioned quite a few times in the book Screening Space, so if you do see it don't hesitate to share your reactions.
October Sky (1999)
It's not an sf movie at all--it's a drama about a few young men who build model rockets in a coal mining town in the 1950s. However, it's quite well made, and one of my favorite movies with the space race as a back drop.
The Right Stuff(1983)
Not sf, obviously, but probably the best movie made about the early days of the space race.
The X-Files: Fight The Future (1998)
If you have the series, the first feature film is an integral part of its mythology (it takes place between seasons five and six).
The X-Files: I Want To Believe (2008)
Fans were pretty divided over the second feature film, but it may be the final word on Mulder and Scully. If you're invested in their characters, it's worth seeing. It's a Monster of the Week type story, but with a strong emphasis on character.