Download the Mind Candy 2 DVD for Amiga demos: http://www.mindcandydvd.com/2/ It's a fantastic DVD that was released about 10-15 years ago as a physical disc and is now available for free. The demos were captured from real hardware and they went out of their way to ensure it was the highest quality capture at the time. It's a trip, and many of the releases have commentary tracks with their creators.

The volume 1 DVD on PC demos is equally good. The blu-ray #3 is great for later 2000's era and early hardware accelerated demos (I don't think this disc is a free download though).