The Osterlind Breakthrough Card System
cost $15
difficulty 1 if you're good at simple arithmetic
(1=easy to do, 2=No sleights, but not so easy, 3=Some sleights used,
4=Advanced sleights used, 5=Suitable for experienced magicians only)
The Breakthrough Card System (BCS) by Richard Osterlind can be downloaded from his site
http://www.osterlindmysteries.com/main.htm
for $15. It is a cyclic system like Si Stebbins: given one card you can work out the next. The big difference is that the cards appear to be completely random (the ebook explains how you can take advantage of this). With Si Stebbins the red-black pattern is immediately obvious. With the BCS there is no discernable pattern: in several places reds and blacks alternate for a few cards, there is one run of six red cards, and in three places there are runs of three black cards.
There have been several attempts to devise card systems (Bart Harding's is one of the best) but most were complex and generally had exceptions which had to be memorised. Or if not, the pattern was obvious. Another big advantage of the BCS is that it cycles: that is, card 1 follows from card 52, so the pack can be given a genuine cut.
This solution to the problem by Richard Osterlind is very clever. You cannot reverse engineer the simple formula from the sequence of cards. It starts with AS, 3S, 7D, 5H, QC, AC, 5S, JH, JC, ... Looks random doesn't it?
The system needs a bit of memory work but not much. It also needs a bit of mental arithmetic; easy enough, but this would not be suitable for the terminally innumerate.
overall 10/10 the perfect solution to a difficult problem