I'm going to highlight a few shoot-em-up games that were created on the classic Atari systems. A few are indie games while most are vintage titles from way back when. All come highly recommended, so break out your Wico joysticks or PC gamepads and get cracking.
Juno First (Atari Age, 2600) is a spectacular translation of the 1983 Konami arcade game, and one of the best indie/homebrew titles on the Atari 2600. Blazingly fast, terrific visuals and even a catchy chiptune song. Really fantastic stuff.
Space Rocks (Atari Age, 2600) is the definitive version of Asteroids, featuring a host of options, including color or monochrome display, vector or solid graphics, 2P versus or co-op, support for Sega Genesis joypads and more. An indie/homebrew classic.
Desert Falcon (Atari, 7800) is an isometric shooter, similar to Zaxxon and Blue Max, where you fly a bird who shoots down enemies while collecting gems and rare items that grant you power-ups. There are nice little touches like the bird hopping on the ground or swimming on water, and the way he gets knocked dizzy when crashing into solid objects. It's classic Atari Corp. weirdness through and through.
Zeppelin (Synapse, 800) is an expansive shooter where you fly a zeppelin armed with guns and bombs. You must shoot enemies, bomb houses, locate keys to unlock gates, and ultimately find dynamite to destroy the enemy lair. The underground caverns are enormous and slightly surreal, with towns, buildings and trees located everywhere, as well as the occasional earthquake. It captured my imagination as a child as few things have.
Blue Max (Synapse, 800) is a stone cold classic of the home computer era. Think of it as a mashup of Zaxxon and River Raid. You fly a biplane, can shoot down enemy planes and bomb ground targets including bridges, cars and buildings. At the end of each stage you must land your vehicle without crashing (and without being bombed on the ground).
Blue Max 2001 (Snapse, 800) is the futuristic sci-fi sequel, with a mirrored viewpoint and a flying saucer. Your goal is to blow up everything that moves while dodging enemy fire. It was less popular than the original but a very good game, and also features one of the great chiptune songs of the 1980s.
Fort Apocalypse (Synapse 800) is a copter shooter that blends elements of Choplifter and Super Cobra with a host of new ideas. It's a very tough game and there's always numerous enemies and challenges as you seek to rescue hostages from underground caverns.
Tail of Beta Lyrae (Datamost, 800) was my favorite shoot-em-up for the Atari home computer, featuring that wonderful Gary Gilbertson chiptune score and brilliant stage design. Each level is randomly generated and highly challenging.
Dropzone (US Gold, 800) is the Archer MacLean classic that totally rips off Defender but does it right. But a very good one, very fast, very colorful, very tough. Explosions are very satisfying and enemies are just relentless.
Stealth (Broderbund, 800) is a 3D shooter where you must destroy a giant tower on an alien world, while dodging various enemies and flying over fuel pods to stay alive. Also known as Landscape (beta) which found its way onto everybody's floppy discs (ahem). The Atari 8-bit did an excellent job with 3D graphics.