I agree with tigsource.com: fraxy is indeed awesome. You can get your own copy over on the developer’s page for the game. This is fast paced arcade action at its finest, basically a series of boss battles strung together with not much purpose beyond getting your blastem on. A couple of things to be aware of: first, don’t neglect the use of speed boost, it’s critical to success against many of the bosses. Second, be sure to grab both the .exe and the english text translation file, and read the readme on how to install the language file. The download links are right at the top of the screen on an otherwise completely asian language website. Also check out places like this, and this (you’ll have to dig around a bit) for more enemies to install.
This is free, windows only, and a pure adrenaline blast of arcade fun.