The 2015 Christmas Sale started at Acousticsamples. They offer a 30% discount plus up to 20% off if you purchase more than one library.
Their activation system even allows you to offer a library to someone, just order a library from the website and offer the serial number.
The 30% discount may be combined with the quantity discount available all year.
* The discounts are available on everything but the Ircam Prepared Piano.