Wagn lets you "follow" individual cards or all cards of a given type. You will be sent an email when other people change a card that you are following.
Whenever you are logged in, the card menus will include a top-level "follow" link. Click that link to follow that card.
Navigate to a Cardtype card, and then follow the same procedure as above.
If you are following a card, the menus link will read "following" instead of "follow". When you mouse over it, it changes to "unfollow". Click that link to unfollow the card or cards.
There is not yet quick interface for this, but you can go to |user's card|+*following to see a Pointer card that indicates which cards a given user is following. When a signed-in user clicks "follow" or "unfollow" anywhere on the site, this card is automatically updated. It can also be edited directly like any Pointer.