For horror/sci-fi, all the usual stuff like Alien, Aliens, Terminator, T2, Event Horizon, Predator, Jason X, etc.
For lesser known stuff,
Lethal Target - takes all the FX footage from one of the Xtro movies and recycles it. C.C. Costigan is pretty good in the lead role. Great exploitation stuff, yippee
Can get sort of deadly dull in spots, but the T&A holds my interest.
Bone Snatcher - small group of people in the desert are attacked by mysterious monster type thing. Good characters and atmosphere.
Breeders (1998) - alien monster attacks hot babes on an all-girl college campus.
Centipede - giant centipede attacks kids in some caves in India. One pretty hot babe. Total cheese fest.
Cerberus - three headed dog chases after Emmanuelle Vaugier and Sebastian Spence. Cheesy but entertaining.
Crash and Burn - android stalks people in a post-apocalyptic shelter. Features a giant robot at the end!
Creature (1985) - Group of people land on Titan and are killed by an alien creature. One of the better sci-fi/horror movies of the '80s.
Dracula 3000 - I'm one of about 4 people who like this, and the other three aren't so sure. A corny Dracula chases Casper Van Dien, Erika Eleniak, and Tiny Lister around a spaceship.
The Flesh Eaters - nice B&W movie about a mad scientists' creation threatening a group of surprisingly well developed characters around an island.
Ghosts of Mars - a fun movie about some weird entities that take over the people on Mars, turning them into rejects from a Mad Max movie. Natasha Henstridge does a great job, and looks hot.
Hellraiser Bloodlines - crazy cool movie that starts out in the future on a space station, then goes back in time to cover the history of the Lament Configuration puzzle box.
Insecticidal - total cheese fest with giant bugs attacking barely-dressed babes in a dorm.
Inseminoid - slow moving but atmospheric horror movie about some people on a far off moon being attacked by an alien monster.
Leprechaun 4 - In Space! Almost beyond a cheese-fest, but I thought it was a load of fun. the little Leprechaun picks off some space Marines aboard their spaceship.
Leviathan - Great Alien-type movie about a creature killing people on underwater mining facility. Excellent characters including Peter Weller, Richard Crenna and Amanda Pays.
Manticore - Nasty critter in the Iraqi desert stalks some Marines. Not bad at all for a Sci-Fi Channel original.
Not of this Earth (1988) - Blood-sucking alien is after Tracy Lords, who's got nothing but her blue bikini to protect her! Totally fun cheese fest.
The Shaft - Killer elevator threatens the residents of Manhattan. Naiomi Watts looks hot. Really fun movie in my opinion.
Sphere - Alien sphere at the bottom of the sea causes problems for Dustin Hoffman, Sharon Stone and Samuel L. Jackson. Good movie, could have been a bit shorter though.
Virus - Alien virus is downloaded from the Mir space station and causes problems for Jamie Lee Curtis and Donald Sutherland. I really liked this one, exciting, mysterious and a little bit scary.