I found a few web-apps that show you the note. The best being Guitar Tuner:
- it pauses after you sing the note
- it tells you which octave (eg. G2 would be 2nd octave on the bottom)
- it tells you the frequency
Another one is Pitch Detector.
It works, but I find it to be quite flickery and it tells you the octaves in cents, which would take another few seconds to calculate, so isn't as easy.
There's a live demo on the site, with a whistled 'happy birthday' you can use to see if it works for you:
There's also Microsoft's Web Audio Tuner, which is currently a demo, but seems to work fine:
- it gives you the note, octave, cents, and pitch
- allows you to alter the base frequency to tune it