From Japan, created by Kenta Cho and available on his ABA Games website are a bunch of shmups, and all with source code. Highlighted here is Noiz2, a game that runs as an applet in any Java-enabled browser.
Noiz2 is an excellent little game that can keep you mesmerized for hours. Your ship is the mouse pointer, move it around the screen to shoot at enemies and avoid their fire. To destroy enemies, position your ship directly underneath one and it will automatically lock-on and fire back. Green stars are bonus items, so collect them to increase your score. Click.
The graphics are very nicely done, but unfortunately there is no sound. The version number indicates that it is still a work in progress, so it's possible that it may get even better. Still, the game is very pretty to look at and fun to play, and if you feel like improving on it, you'll find the source code available for it here. Besides Noiz2, there are many other shmups available at the author's site that run on a variety of platforms.
Remember that this game is a Java applet so patience is needed when loading it in the browser.