What did computers use for output before monitors?

The first computers used punch cards or lights. Wikipedia says for ENIAC:

an IBM card punch was used for output

The German Wikipedia-article about Zuse's Z3 (the first apparatus that can be called computer) says:

einer Tastatur mit Lampenfeld für Ein- und Ausgabe von Zahlen und manuelle Steuerung von Berechnungen

That means: it had a keyboard with lights for input and output (programs were read from punch cards, as NobbZ said).

As computers got more common, they used often the already existent teletypes, that means output was made through printing text on paper. Until today, Unix-Terminal emulate to some degree teletypes.

EDIT: You asked specifically about PCs. First PCs came up in the 50s and had different methods for output. The IBM 610 used a keyboard for input and an electric typewriter for output. Also, some other early PCs used printers, i.e. Olivetti Programma 101. The Simon or the Kenbak-1 used lamps for output.

Monitors seem to come up 20 years later, in the 70s. One early model with monitor is the Datapoint 2200 from 1970. And naturally, the milestone Xerox Alto.

More infos about history of PCs at Wikipedia (as usual).

You may want to check out the Computer History museum's timeline or http://www.thocp.net/index.html

It's been a while since I spent much time there but it is very interesting and may be helpful. It will probably give you some computer names that you could then research to find out how people interacted with them.

Before monitors, computers used Punch Cards, Paper Tape, Banks of Lights, Fan Fold Paper, and Bells for output.

When I was going to school, we submitted our programs to the sysadmin (input) on punch cards and the output was printed on fanfold paper.

To answer your specific questions:

• MARK I (1944): Used a 24 channel punched paper tape for input and output. Or for alternate manual data entry, 60 sets of 24 switches could be used.
• ENIAC (1946): Used a card reader for input, and a card punch for output.
• Simon (1950): Used a five hole tape for input and five lights for output.
• UNIVAC (1951) Used a keyboard for input and register lights for output.
• Xerox Alto (1973): Used mouse and keyboard for input and a Monitor for output.
• MITS Altair 8800 (1975): Used an ASCII keyboard for input and a '32 character alphanumeric display terminal' for output.

To illustrate an alternate input / output method in use near the dawn of the PC:

• Pong (1975): Used two knobs for input and a TV for output.

Some other early computers:

• Zuse Z1 (1938)
• Zuse Z2 (1939)
• Zuse Z3 (1941): Used punched 35 mm film stock.
• Atanasoff–Berry Computer (1942)
• Colossus Mark 1 & 2 (1944): Used patch cables and switches and paper tape for input and banks of lights for output.
• Zuse Z4 (1945): Used punched 35 mm film stock.
• Manchester Small-Scale Experimental Machine (Baby) (1948)
• Modified ENIAC (1948)
• EDSAC (1949)
• CSIRAC (1949)

And even before all that:

• Mechanical Calculator (Pascal) (1642)
• Jacquard Loom (1801)
• Scheutzian Calculation Engine (based on the design of the Difference Engine) (1843)
• Arithmometer (1851)
• Difference Engine (1859): As @artistoex pointed out, used a bell and a typewriter.

And even before that, as @WernerCD and @artistoex pointed out, calculation assistance has been going on for quite a while:

• Tally Sticks (33,000 BC)
• Counting Pebbles (Used to keep track of sheep, ~8000 BC)
• Egyptian Abacus (3,000 BC)
• Abacus (2400 BC)
• Antikythera Mechanism (100 BC)
• Counting Rods (~100 AD)
• Planisphere (1000 AD)