Star Opus I is a nicely-implemented shmup with Asteroids-style motion control and shooting. There are a lot of such games, of course; what makes Star Opus stand out is the sheer number of potential enemies and the interesting ways in which they are used and combined in 25 different levels to pose different challenges with each level.
Visuals are vector-graphics style in old-school monitor green, but with nice floating visuals as background; music is a pleasant and unobtrusive electronic score. It's not that there's anything particularly unique here, but the package as a whole is pleasure and challenging to play.
Star Opus appears to have been a commercial indie venture, but the developers have released it as freeware -- though they threaten an additional 25 levels in future, presumably for actual money. It's worth checking out, however, if the game style is one that appeals to you.