I always recommend
Music Math. It's freeware, gives note / freq / MIDI values, as well as handy tempo to Hz / ms, and other features. An all-in-one app that I always have open when doing anything music.
Edit: MusicMath is not a recommendation for replacing the app mentioned above, just a "hey also" suggestion. Note2Frequency is much more informational and accurate for it's particular function, while MusicMath has rounded values for Hz, and no overtones or playback option. Good find!