Arduino boards are great for making lights blink or controlling motors, but if you want to show output from it on a monitor, this task can be tricky. To make things less tricky, the NovaVGA Arduino VGA shield allows you to control a VGA monitor with only three pins acting as a Serial Peripheral Interface (SPI).
In use, this board takes commands from an Arduino (or any SPI-compatible microcontroller) in a 160×120 resolution. It then converts it to a 640×480 resolution that should be understood by nearly any monitor you find to use. To help you get started programming (or if you just want to enjoy a casual game), seller Micronova has an Arduino library available, and includes examples including a color palette, a Mandlebrot set visualization, Tetris, and a text console.
One interesting thing to note about this board is that it appears to use an R-2R resistor ladder to accomplish digital to analog conversion (note the series of resistors next to the actual port). You can read an explanation of this technique on Wikipedia, or there’s a more “hands-on” approach to VGA output using a series of resistors on Hackaday.io for another use example.