An Alpha version of Mythical Wings has been released over on the Shmups Forum and already it has a lot of cool features. It will eventually be released open source, and will serve as a 'kit' you can use to put together your own vertically scrolling shooter for free.
"The game feels like oldschool games (Raiden and such), but has some modern elements (sometimes bullet hell, suicide bullets, combos, random levels).
The levels are text based and are loaded from bottom to top (vertical scrolling), this allows anyone with a text editor to change level, generate levels using a script, copy and paste levels. Anyone can share his levels / game online (host them somewhere) and anyone else can play them, no installation required.
The code itself is pure JavaScript and some JSON files for the config, both can be edited using some text editor. You do not need to compile the JavaScript, so editing text files and refreshing the browser page will suffice. So all you need for basic games is modify text files and make some graphics with a paint like software or whatever.
For advanced stuff like custom enemy movement, custom weapons, you must be able to script. For the moment the engine is not online but it runs around 60 FPS no problem, maybe a bit less for extreme bullet hell situations."