PocketMatrix

It's a neat feature, but currently it's just used for rootkits and malware to hide in. Additionally, it is very easy lost in transferring files. OTOH, a resource fork like ADS is done in OS X, and done pretty well.

If you ever download a file with IE, and have seen the dialog box "This file came from the Internet" at the bottom, that would be IE appending an ADS telling the shell that it's from the tubes.