Natasha Romanoff will always be Marvel's most famous Black Widow, and Yelena Belova will likely become more prominent in the comics after making her Marvel Cinematic Universe when Black Widow hits theaters. However, Marvel's Ultimate Universe gave birth to another Widow, Monica Chang.
Like the other Widows, Monica was a leading member of S.H.I.E.L.D., the Avengers and the Ultimates. Eventually, she even became popular enough to follow in the footsteps of other Ultimate Universe favorites like Miles Morales and make her way into the main Marvel Universe.
Monica first appeared in 2009's Ultimate Comics: Avengers #3, by Mark Millar and Carlos Pacheco, Ultimate Comics: Avengers consisted of a series of three mini-series, all written by Millar, that focused on the Ultimate Universe Avengers, S.H.I.E.L.D.'s black-ops superhuman strike team that served as a covert alternative to the Ultimates, the universe's main, public super-team. The black-ops Avengers concept was first introduced in the first volume of The Ultimates, also written by Millar, and had previously introduced the Ultimate versions of Romanoff and Hawkeye.
The first miniseries, subtitled "Next Generation" saw Nick Fury assembling a new iteration of the Avengers to combat the Red Skull and apprehend Captain America, who had gone rogue in an attempt to do the same. Much to Fury's chagrin, S.H.I.E.L.D. director Carol Danvers demanded Monica be included on the roster, partially to monitor Fury. It's revealed that Monica and Fury were briefly married, and the two still maintained a level of mutual respect and affection, even though their divorce came about because of some particularly slimy actions on Fury's part.
Despite their shared identities, Monica bears no connection to the prior Black Widow, who in the Ultimate Universe was a ruthless traitor. Given that she has no connection to the Red Room her training and abilities are different than Romanoff's, she still has the Widow's usual skill in combat and espionage. In her first appearance, Monica wore a Stark-tech super-suit that greatly enhanced her physical abilities. Early on, Monica was notable for her abrasive attitude, even often mocking her teammates about her use of a traitor's codename, but she mellowed out as time went on. By the end of Avengers vs. New Ultimates, it was also revealed that she has a baby son, Julius.
One of the few constants on the Avengers' ever-changing roster, Monica became one of the series' breakout characters and this led to her being featured in more and more Ultimate comics. With Fury reinstated as director of S.H.I.E.L.D. she was promoted to the Ultimates team in the subsequent ongoing Ultimate Comics: The Ultimates. In Ultimate Comics: The Ultimates #6, by Jonathan Hickman, Brandon Petersen, Esad Ribic, John Rauch, Edgar Delgado, and VC's Clayton Cowles, Fury was confirmed to be Julius' father. After Fury was ousted from his position and Captain America and Monica exposed his replacement as corrupt, Captain America, who had been elected president, named Monica the new director of S.H.I.E.L.D. After the destructive events of Galactus' attack in "Cataclysm," she was removed and bounced around the Ultimate Universe until she was killed by the Green Goblin in 2014's Miles Morales: Ultimate Spider-Man #3 by Brian Michael Bendis, David Marquez, Justin Ponsor, and Cory Petit.
Shortly before her death, Sam Humphries and André Lima Araújo introduced the Marvel Universe's Monica Chang in 2013's Avengers A.I. #1. This Monica was the chief of S.H.I.E.L.D.'s artificial intelligence division and was a founding member of the titular team. However, she hasn't been a major character outside of the short-lived series and still hasn't followed in her Ultimate counterpart's footsteps. But given the increasing importance of the Black Widow mantle in the Marvel Universe, it may very well only be a matter of time until takes on that iconic role.