Has anyone mentioned The 6th Day, yet? Or Timecop?
I watched Timecop a while back and The 6th Day last night. They're both daft but they're still enjoyable and in their own ways they're actually quite good, kinda like The Fifth Element - daft but incredibly enjoyable.
It seems to me that Arnie really enjoys his SF films (I don't think I've seen a film with him in that isn't SF in some way or other).
EDIT: Oooh, I've watched a lot of films over the past few months which I think deserve mentions (don't think they've been posted yet). I know a number of them are quite recent, but that doesn't mean they shouldn't be watched:
9 - talking puppets in a post-apocalyptic world. Some nice ideas.
1408 - contains best Samuel L. Jackson live ever, "It's an evil -ing room". King adaptation.
Akira - some nice Japanese post-apocalyptic stuffs.
Back to the Future Trilogy - I may be wrong, but I didn't see this mentioned as I scanned back through the thread.
Blade Trilogy - same goes for this. It lost direction by the third, but the first is brilliant.
Cronos - in this world of Twilight, a totally different view of Vampires is nice to see. One of Del Toro's earliest films.
Equilibrium - haters be haters. A dystopian society with some superb action sequences.
Gamer - playing FPS with criminals. A very interesting idea.
Jumper - nothing spectacular, but seeing someone jumping around the world at whim is amusing. Plus it has Samuel L. Jackson in it!
Pandorum - space horror with Dennis Quaid. I can't find enough things set on abandoned spaceships. Wonderful twist.
Silent Running - classic, slow SF. Biodomes, insanity, and poker with three robots.
Stargate - the film that started the series. Quite fun.
Hitchhikers Guide to the Galaxy - someone must have mentioned this! Sure, it's not as great as the books, but it's a good attempt.
Unbreakable - Bruce Willis and Samuel L. Jackson in a non-comformist superhero film.
And then there are things like Primer, Riddick, Terminator and various roboty things which have already been mentioned but I'd like to add my backing to.
EDIT2: OK, more.
David Lynch's Dune, whilst strange, was a nice step in the right direction.
2010, the sequel to 2001 is good fun. Not entirely necessary, but still worth a watch.
The 1995 film of Casper still gets me and I recommend watching it if only for the beautiful piano theme, One Last Wish (Horner at his best).
Coraline is a surprisingly creepy adaptation of the Gaiman novel, but holds wonderful rewatchability (yeah, it's a word now).
Daybreakers is a look at a future ruled by vampires whose supply of blood is running out - a nice concept that I've not seen before.
I'm going to throw out Independence Day for it's sheer bad-assery.
The Mummy is a film I've re-watched many times, yet it still makes me jump. The sequel, The Mummy Returns, was alright, but the third one was dire... I'm not even going to name it.
Sleepy Hollow is always good fun, as is another film with Christopher Walken, The Stepford Wives.
And a zombie film that doesn't really take itself seriously, but it fantastically brilliant nonetheless - Zombieland!