What is the name of your state (only U.S. law)?
Virginia,
Can I legally acess my wifes work email account if she left it up and open on our home computer?
Her company is based in Washington DC and we live in Virginia. It is not a goverment company.
You've already gotten a number of responses of the ethics involved. But that wasn't your question. Unfortunately, what the law requires is not always the moral thing.
You need to check the law in Virginia. In my state, the answer is quite different than the responses you've gotten. I was told that if my ex gave me access to the account and did not change the password, that I could not be charged for reading her email. If, OTOH, she changed the password, it would be illegal for me to access the account.
Regardless, her employer would have cause for action against her, not you. You have no legal obligation to protect her employer's information.
Of course, beware that it IS one of those things that could backfire badly on you.