Quite an ingenious simple little method to hide files on an ntfs disk: alternate data streams. This article on Security Focus makes it look a little more difficult than it is, due to the author going through the effort of describing breaking into a machine to set an ADS on a few hidden files. LNS and LADS are two tools to scan a disk for ADS…although they are certainly not swift in their scans.

Update: An ADS tutorial from STC