Radical Spectrum: Volume 1
I would really like to recommend this game as it feels like a spiritual successor to Yars' Revenge. Like Yars, defeating the enemies requires a two step process: clear a vertical line in the grid and then shoot the boss though the gap you created. The only way to get a shot is to gather three globs of a single color without touching another color. So the core of the game is deciding which color to gather and deciding when to shoot the canon.
Meanwhile, the enemy occationally drops a black glob, which will end the game if it reaches the floor. The canon can destroy the glob, but it can take a while to collect more colors for another shot. A made shot can move the player to the next screen and a missed shot can doom the player to destruction. This is very much the same sort of tension you feel in Yars when preparing to make the final shot. It's a good design and more challenging that it first looks.
Radical Spectrum: Volume 1 isn't quite as compeling as Yars' Revenge, however. (For a glipse into that game's design, read The story of Yars' Revenge is a journey back to a lost world of video games [www.polygon.com]
.) It's just too slow and repeative. But for the price, it's worth trying out if you enjoy thinking about action game mechanics.