The biggest site when it comes to do demos for all platforms, like the Amiga, MSX, Commodore 64 and PC, is Pouet.net. Visit them here. We’ll add more links in the future.
Old School Game Blog (blog about retro gaming, but also some posts about demos)
Pouet.net (superb demo portal with new releases, discussions and so forth)
Solskogen (the home of the Norwegian demo-party)