What are Twitter flashcards? You may not know, because they were introduced very recently.
To the left is a screenshot. Basically, whenever you scroll over someone’s name, whether it be in a mention or before the actual tweet, one of these will pop up. You can follow the person, unfollow, and see their statistics as shown to the left. It’s a very helpful tool since you don’t have to go directly to their user page to view all this.
Some apps, like Echofon for Firefox, already have this feature, but it’s not as good.