Check out the Propellerheads site. Recycle is the program that creates them.
http://www.propellerheads.se/
Also, if you check out
www.soundsonline.com you will see that the REX2 file format is becoming more popular in commercial sample cds. They even have a category for "Reason/REX" file format.
It's not just for Reason. Logic, DP, and Cubase also can read rex files directly. You MAKE rex files with Recycle. Recyle will also export them at a specified tempo as aif or wav files to import them to PT.
There is a similar process called "groove control" by Ilio, I believe. Check out their site as well.
www.ilio.com
The ACID format is along the same lines, too, but not as many (if any) programs other than ACID can use them. Check out ACID loops...tons of manufacturers. They have them at soundsonline, too.