1. Primer (2004)
Synopsis: Two engineers working in their garage accidentally create a time machine, and must grapple with the technical and moral implications of their discovery.
Why it's great: It doesn't get more low-budget than this movie. Shane Carruth wrote, directed, starred, edited, and even scored the movie all by himself. The low-budget actually works in Primer's favor. Usually time machines are mysterious objects that dazzle with special effects. With Primer, we see what a real time machine would probably be like. This movie is a surprisingly realistic and scientifically accurate portrayal of time travel, and has some great twists that will keep you guessing. Then you'll want to watch the movie again to figure out what you just saw.
2. Monsters (2010)
Synopsis: A photojournalist and his boss' fiance struggle to get from an alien-infested Mexico back to the US. Along the way, they fall in love.
Budget: Less than $500,000
Why it's great: Gareth Edwards not only wrote and directed this monster movie, but he also produced the special effects. He used guerrilla film techniques, shot in locations without authorization, and often using local people as actors for most of the roles. The movie succeeds by portraying a believable world dealing with an extraordinary event such as an alien invasion. It feels and looks real, and focuses more on the two main characters making their way through an unfamiliar world. And yes, the monsters are pretty cool.
3. Cube (1997)
Synopsis: Five people find themselves trapped in a maze of identical cube-like rooms, some filled with death traps, and must find their way out.
Why it's great: Even though it's technically the same cubical set over and over again, there is a genuine fear of the unknown each time the characters enter a new cube. This movie is horrifying in the gruesome deaths, but also in the way it shows what depths people are capable of in their fight for survival. Cube ruthlessly breaks all our preconceived notions by setting up character archetypes, then shocks us by what directions they take.
4. Pi (1998)
Synopsis: A mathematician stumbles across a number that could be the key to extraordinary wealth, spiritual enlightenment, or madness.
Why it's great: There aren't many movies about mathematics, but Pi will make you wonder why. A dark psychological thriller, this movie focuses on obsession and conspiracies over spaceships. Yet it's a powerful film about a single man who finds himself the target of religious zealots, Wall Street tycoons, his own determination to understand how his discovery affects the universe.
5. Mad Max (1979)
Synopsis: In a dystopian future Australia, ruthless gangs roam the roads, and a policeman nicknamed Mad Max seeks revenge for the death of his family.
Why it's great: It's hard to believe this modest movie spawned a blockbuster series. Mad Max was followed by Mad Max 2: The Road Warrior, and Mad Max: Beyond Thunderdome. At only $380,000 in Australian dollars (which would be about $566,200 in US dollars or $1,870,954 USD adjusted for inflation), Mad Max had such a low budget that they could only afford one leather jacket for Max to wear. Yet the movie captured a sense of nihilism and desperation, and ushered in a post-apocalyptic genre that is still flourishing today.
Any other sci-fi movies that you know of which have low budgets but high entertainment?