Most non-IFR, generic Sirf GPS chipsets, as used in lots of mobile phones and even car satnav devices, also perform the required corrections from ellipsoid altitude to local mean sea level altitude.
In fact the NMEA sentence specification requires such corrections to be performed as the only altitude output should be with regards to local mean sea level. There is another, separate output which is supposed to tell the receiving software what correction has been applied, in case they want to reverse it and actually use ellipsoid altitude. This makes it quite easy to detect when a device has actually given you ellipsoid altitude which on its own is quite useless for flight.
My mobile phone produces a GPS altitude within +/- 20 feet of the altimeter reading.