Set Amazon S3 Bucket as 'Public' by default

Select the bucket within your Amazon S3 console, hit 'Properties', and then hit 'Edit Bucket Policy' under the 'Permissions' tab. Then, paste in this text:

{ 
    "Version": "2008-10-17", 
    "Id": "924a2348-de0e-43aa-bb06-83adbcd1db22", 
    "Statement": [ 
        { "Sid": "PublicReadForGetBucketObjects", 
        "Effect": "Allow", 
        "Principal": { 
            "AWS": "*" 
        }, 
        "Action": "s3:GetObject", 
        "Resource": "arn:aws:s3:::BUCKETNAME/*" 
        }
    ]
}

Source: http://www.bdesham.info/2011/06/building-and-hosting-a-static-site/