GoG.com will exchange you $3 (or local equivalent) for a turn at the Indie Piñata which includes games varying in price from $9.99 to $44.99. While the number of Piñata you can purchase isn’t limited, you can only put 25 Indie Piñata in your cart for checkout.
GoG will attempt to give you a game you don’t own, but if you have all the games you can end up with duplicates which you can gift to friends.
If you don’t like the randomness or just want to find games on offer, GoG is also running deals up to 80% off their usual price at the same time for those who like to know what they are buying.
The Indie Piñata promotion runs until the 18th September.