I have always found NTFS Alternate Data Streams interesting. It is an easy way to hide metadata from users. I wrote a utility called ADSFile as a .NET Wrapper for the Win32 API calls.
Someone asked for me to provide the ability to enumerate data streams into ADSFile. It 's not as straightforward as I thought because the only way I found is to use the BackupRead and BackupSeek APIs to read and find the stream headers. Code is here: gotdotnet.