- Goblin Engineer Membership Card
- Binds when picked up
- Unique
- Item Level 0
- "A member in good standing of the Goblin Experimental Engineering Korporation."
You need to own this item (on your person, or in the bank) in order to get the Goblin Engineering trainers to give you access to the secret recipes for items requiring Goblin Engineering.
Once you acquire this card, it will expire after 14 "days played" (not after just 14 real days).
Source
To initially acquire this card, you have to specialize in Goblin Engineering. After that, you can get it renewed by the master goblin engineering trainer.
See also: Goblin Engineering
Fun feature
The "Goblin Experimental Engineering Korporation" mentioned on the card shortens to "GEEK". This seems to be intended (compare 
[Gnome Engineer Membership Card]).