I agree with eldargal. Black Widow knows what she's done and doesn't shy away from it. She admits it and tries to fix things. The way she goes about redeeming herself isn't always the best, and she doesn't seem to actually believe she can do enough to wipe her slate clean, but she tries and that counts for quite a bit.
It's a straight that Iron Man sometimes shares with her. Unfortunately for him he has a monumental ego that frequently gets in the way and sends him sliding backwards. But wasn't there a moment in one of the What-If comics where Stark very briefly snatched up Mjolnir and successfully fended off an attack to defend Thor (much to everyone's astonishment, he was trying to use it to stop a skid) and then got egoistic and promptly had it nearly land on his foot?*
*It is entirely possible I am confusing this was a particularly well made fan comic, I've never regularly followed Marvel and have no idea where this was from