Old question, but in the comments of the accepted answer, I believe Nate Eldredge is correct and littleadv is incorrect. Nate copied the actual quote from the IRS guidelines, quoted below:
An expense is ordinary if it is common and accepted in your trade, business, or profession. An expense is necessary if it is appropriate and helpful to your business. An expense doesn't have to be required to be considered necessary.
Noise cancelling headphones certainly count as "appropriate and helpful to your business" in the software industry, especially with the trend of open office layouts. And because of the ubiquitous distractions inherent in the aforementioned office space, noise cancelling headphones are becoming quite "common and accepted" for use by developers.
I'd be more hesitant about the keyboard and monitor, as presumably the employer is providing those already. As using your own could be said to just be a personal preference over those provided, the argument that providing your own version is "appropriate and helpful" is a little more shaky.
I am not a tax lawyer, so don't come after me if you get audited, but my guess from reading the actual IRS guidelines is noise cancelling headphones: probably, keyboard and monitor: maybe.