Shmups Forum
The most important resource you'll find for anything shmup and DDP related. Folks here can answer just about anything and there is an archive of many years' worth of discussions and strategies.

Cave
The developers of the phenomenal DDP series amongst other titles.

Peaceful Death
My shmup art site, replete with DDP art, of course.

My DDP:DOJ FAQ
My DDP:DFK FAQ
My DDP:SDOJ FAQ

I plan on adding more here. For now, this will have to do.