I've counted along with things like the gas (petrol) pump dispensers like that for years. Good practice for any musioian when you are doing something besides "just practicing".

If you live in an area with tunnels and commute to work, try continuing to sing a song that is on the radio when you enter the tunnel -- and see how close you are to the track when you come out of the tunnel and radio reception is restored.

When at home and playing the "Radio Game" -- which is simp;ly tuning in a station and attempting to find the key, chords, etc. to whatever musicis playing and play along with it in realtime, great ear training excercise -- try turning down the Volume on a song altogether at some point and keep counting and singing the song. Are you dead on the same place as the song when you turn the Volume back up?
The same can be done with any source, CD, tape, etc.

--Mac