The American forums have a list of voucher codes for the 20th Anniversary event, but we don't appear to have one here (I could be wrong)...
The American list is here, but most of their codes only work on the American .net servers
Information about Codes
Codes are NOT case-sensitive - they should work in upper- or lower-case.
To enter the code, go to the game homepage, and click the 'Voucher' button on the menu. You can then enter the code into the box at the bottom of the page. You will be told if a code is invalid (or if you've already used it) before you click the 'Submit' button.
The event codes are usually hidden on the TSO Homepage, Facebook, Twitter or Forums
Colour Codes
RED: Code has now expired, and can no longer be used
GREEN: Code is currently valid
Event Adventure Codes
Two 'adventure' codes are available. These are supplied only as a bug workaround. If you cannot complete an adventure quest, or you have accidentally cancelled the adventure, but the quest is still active, then you can use these codes to give you a 'spare' adventure. It is recommended that you only use these codes if you really need to:
Old Ruins: 382-6c4d-f2a4 (from 2nd July 2013)
The Shaman: c3c-2da4-9e94 (from 2nd July 2013)
Event Codes
There will be 20 codes for the Anniversary Event. These are rumoured to be one per day, except at weekends, but this is not confirmed. Each code is valid for about 36 hours from the moment it is made available. Check back here every day to find the new code.
NOTE: Expired codes are now hidden in this spoiler tag. Valid codes are below.
Spoiler
18th Code: D34-7565-A6E4Provides 2x Mother Tree Ritual buffs (3x output for 20 hours)
Message: You received a mysterious item from an ancient culture, it would seem it can be used to speed up your production..
Released early on Sunday 21st July 2013 (Expiry is probably Tuesday)
Code was supplied in a clickable image (like the 3rd code), on the homepage of the site