Arm Cannon - Your regular weapon, simply press B to fire. You can have three shots on the screen at a time.
Magnet Missile - Magnet-shaped projectiles that fly quickly and curve to hit a target. Very useful against hard-to-hit enemies that may try to attack you from above.
Hard Knuckle - Slow-moving fist-shaped projectile. Hard to use (no pun intended) due to its slow speed, but very powerful. Can destroy walls your arm cannon can't.
Top Spin - Mega Man spins like a wild man, damaging whatever he touches, and sometimes himself. Due to its unpredictability and general uselessness, I don't recommend using this weapon except for in certain circumstances.
Shadow Blade - A short-ranged throwing star that can be somewhat aimed. The aiming makes it useful, but it lacks range. Still, it's useful in many situations.
Spark Shock - A fast-moving spark is fired forward. Pretty powerful, too. There are a number of situations where this will be useful.
Search Snake - Snakes will crawl along the floor, damaging the first target they hit. Very useful for those hard-to-hit ground-based foes who may lurk below the range of your Arm Cannon.
Gemini Laser - A blue laser will be fired, which will bounce off walls until it hits a targer, or until it eventually fades away. This is a powerful weapon, but only one can be on the screen at a time. For best results, use at close range, and don't miss the first shot.
Needle Cannon - Mega Man can rapid fire needle-shaped projectiles. If there's an enemy weak against rapid-firing, this will be your weapon of choice!