Action cards can be a powerful tool in Twilight Imperium. They are generally obtained during the Status Phase, and so you start the game without any, There are other ways (such as the secondary abilities of certain Strategy cards) that may also provide Action cards.
Action cards are plentiful and can apply to a myriad of situations. Here I will go through the anatomy of an Action card, followed by an index of all the Action cards in the game in alphabetical order, with scans and any special notes for each card (such as number in the deck, expansion releases/changes, etc).
The timing of cards can be important. Strictly speaking, Action cards should be announced before they are played and, if more than one player is playing action cards, they are executed in initiative order. I have yet to see the play order become an issue, but have seen several threads online wherein the problem came about that somebody played 'Emergency Repairs' and another player played 'Direct Hit'. I don't want to go into this as other sites have done so already, but I do want to point out that the Sabotage card is different in that it is not reliant on initiative order - you simply play it in response to another Action card you wish to cancel.
To start us off, let me showcase my Photoshop skills with this flowchart explaining the anatomy of a card:
Following is an alphabetized index of cards. Under each entry I have listed whether the card came with the original game or an expansion, number of cards in the deck, and any special notes that occurred to me.