All public EBS snapshots appear in the EC2 > Snapshots section in the AWS UI. Toggle the dropdown in the top right of the table to "Public" and you'll see them. Sort by size and you'll get some interesting looking ones at the top.
For EBS, step 1 is reading the docs, step 2 is cutpasting a documentation example.
For S3 I'm not sure how people are building their lists. AFAIK the API provides no enumeration. So this is possibly something coming from web crawl data (e.g. common crawl)
It reminds me a bit of old time cdroms.