YouTube constantly has free movies on offer. The titles cycle in and out, but there is always something worthwhile to watch if you can scroll through the dozens of options. This month, we realized there's an abundance of classic horror — most of it sci-fi horror — on the website, particularly the first films in what would later become long-running franchises.

So if you're eager to start from the beginning, want to have something on in the background or just love rewatching these particular films, check out the following origin points for many famous series.

'Alien' (1979)

Ridley Scott's masterpiece still ranks at the very top of the list for both, "the greatest sci-fi film ever made" and "the greatest horror film ever made." Hell, even the trailer might be the best one ever made. This is just pure genius through and through — the casting, the H.R. Giger designs, the iconic sounds of the ship — and it all kicked off a mega property that eventually merged with the "Predator" series. YouTube also has the first "Alien vs. Predator," if you want to keep the good times rolling without paying a dime.

'The Terminator' (1984)

James Cameron's original idea for "The Terminator" was quite different to the one we're familiar with. Allegedly, it was a much smaller and sleeker robot who used stealth first, then that morphed into a hulking bruiser/unstoppable killing machine played by OJ Simpson, and then it (thankfully) turned into the Arnold Schwarzenegger vehicle we all know and love.

The franchise would quickly trade in its horror roots for pure action after this first movie, but as long as you just stick to the first two Terminator films you're guaranteed to have a terrific time.

'Frankenstein' (1931)

Many of the best, most classic Universal monsters are on YouTube for free right now. The studio was essentially built upon the shoulders of these creature features, and while vampires and mummies are more fantasy/spiritual/folklore, there isn't a better place to start for science fiction tales than "Frankenstein."

This movie would spawn a whole franchise in which Frankenstein's monster would get a wife, meet the Wolf Man, get parodied by Mel Brooks and see countless reboots (including one starring Robert de Niro). If you want to see them all, here are some more free links:

• "Dracula" (1931)

• "The Mummy" (1932)

• "The Wolf Man" (1941)

'Signs' (2002)

This is the last good movie M. Night Shyamalan made before his career went south with a slew of total duds (he has since rectified this, recently making movies people like again, such as "Old" and "Split"). Back when Mel Gibson was unproblematic and Joaquin Phoenix wasn't a household name, "Signs" was a definitive piece in showing how absolutely terrifying aliens can be, and how effectively a well-timed pop-up scare can fuel a story.

It sort of peters out of momentum by the end, but there's no denying "Signs" was a big hit — it had some great moments and would end up being the main spoof subject in "Scary Movie 3."

'Child's Play' (1988)

Okay, this last one is a cheat, I'm sorry. The Chucky movies are not science fiction — the soul of a serial killer gets trapped inside of a doll through the use of a voodoo spell (yeah, we know) — but YouTube has several of the sequels also online for free, so we wanted to include it.

There's an entire world of spin-offs, TV shows, reboots, merchandise, video games, character and lore related to this grotesque little doll, but if you just want to see what all the fuss is about, then check out the original that started it all.

[Image credit: NBC Universal/YouTube]