RetroVaders is a fast-paced version of the arcade classic, Space Invaders.
The program is highly configurable. It supports three styles (Classic, Atari 2600 and its own RetroRaiders), and allows you to choose from three backgrounds. You get to decide how many bunkers you have available (0 - 4), too. You're able to play in full colour, or mono for that early arcade feel, and the program can run full-screen or in a window.
Whichever options you choose, though, surviving more than a minute or two will be a challenge. Your guns fire slower, and the aliens move more quickly than we remember: it takes quite some practice to be able to survive even a single level.
The attention to detail means you'll at least have fun trying, though. Everything from the opening screen graphics to the bleeping sound effects bear a strong resemblance to the original game, making RetroVaders an entertainingly nostalgic, if also slightly frustrating way to pass the time.