Steam is notorious for its twice (and sometimes three time) annual sales. The sale always features deep discounts on popular titles as well as indie games you may never have known existed.
This years first day of Holiday sale Features Skyrim for 19.99 and The Witcher Franchise for both of which are new and a great value. It also features Besiege which is crazy fun, for $5.59 (30%off) and Killing Floor 2 indie titles that for the price point they are at can not be passed up. This years steam sale as always will run for 7 days.