The Black Widow AKA Чёрная вдова the Russian secret agent is all set to make her live action film debut as a supporting character in the upcoming Iron Man 2. The character first appeared back in 1964 in Tales of Suspense #52. Since then she has had just a few on screen appearances, mostly in the last couple of years.
Natalia the Black Widow first appeared in the Marvel Super Heroes show during the Iron Man segments way back in 1966.
It would be 40 years before Natalia would get another moment on screen. She was a main cast member in The Ultimate Avengers and Ultimate Avengers 2 movies.
The fledgling romance with Captain America hinted at in the Ultimate Avengers was consummated (though not on screen) in 2008's Next Avengers: Heroes of Tomorrow in which the Cap and the Widow's kid James Rogers led a new team of Avengers. The lovebirds were shown in a flashback.
In 2009, the Super Hero Squad featured Black Widow in the episode, "Deadly Is The Black Widow's Bite!"
But it will most likely be Iron Man 2 that brings Black Widow to the mainstream. She was featured prominently in the Iron Man trailers and appears to have a significant role most likely leading to her inclusion in the upcoming Avengers movie.
Update July 17, 2011
Black Widow has also become a mainstay of the animated series Avengers: Earth's Mightiest Heroes.
And the Widow will return in 2012's live action Avengers.