I know there’s a bunch of shmup fans out there. If you fit into this group, you should take a look at Endless Fire, a finely-crafted article reguarding the history of the shoot-em-up genre. It covers a great collection of games and tracks them through the begining of video game history through the newest games of today.
The article also includes many screenshots in order to both show you the genre’s graphical progression, but all provide you with some eye candy. It will also give you a great look into one of classic gaming, most classic genres and may give you inspiration to look for some games you haven’t tried yet. (Just Google ones that interest you!)