Works nearly site-wide with few exceptions.
Coupon/voucher has no threshold and appears to work in all region, including the UK, EU, and most Asian regions (including China?) Let everyone know in the comments if it doesn't work in any particular and a certain game.
Obviously certain AAA titles are excluded from this, including Dragon Age: Inquisition in North America and The Crew.
Coupon/voucher has a $10 threshold. It works on most titles sitewide, though it does exclude select pre-order titles, notably titles from EA. Works in all region, including the UK, EU, and most Asian regions.
For our fellow Steam gamers visiting from abroad who doesn't want to fiddle with robot translation:
Use Coupon: NOVEMB-ERGMGX-20XOFF