Some smartphones have a barometer and there are plenty of barometer/altimeter apps at their respective stores. No idea how accurate they are but used along side a GPS they might be useful.
barometric altimeters are generally pretty accurate, its just that 99.99% of the population doesn't know how to use one, and they vary as weather changes.
For automotive/motorcycling applications where you travel hundreds of miles a day, barometric altimeters need a lot of periodic re-calibration to remain accurate. The part that most people don't understand is altimeters are all calibrated to what's called a 'standard atmosphere'. It defines the expected air density and temperature at various altitudes. Problem with that is the actual conditions are very rarely the same as the 'standard atmosphere', so the altimeter reads incorrectly, even though its working exactly as it was designed to work. For example, if you're riding one of the CA passes at 7,500 ft. the standard atmosphere conditions state that the temperature at 7.5k ft. should be right about at freezing, 32F or 0C. When you're riding there the actual temp is probably in the 70-80F range, so the air is far less dense than the altimeter is calibrated for. It will read almost 10% off in that case, because the 80F air is only 90% as dense as 32F air at that pressure.
So when you're riding, you have to mentally calculate the altitude based on how much you know the altimeter will be off, and it varies with altitude and temperature, so its more work than its worth. If you're hiking and things change more slowly, its do-able.
GPS altitude doesn't have any of those problems, but it has the problem Mark mentioned. The people who have posted that they got elevation accuracy to 1m were just coincidence, but its usually good to within 30ft/10m or so on average. For motorcycling you generally don't need surveyor quality readings, if its within 30ft its probably good enough.