See.....

http://www.cmac.com/mt/databo.../ocxo.html

Not a device you would see in everyday equipment, but in freq counters, RF gear - anything that needs a more precise reference than your average crystal. They quote 10ppb(parts per billion) whereas the usual spec might be around 30ppm(parts per million). They draw a bit of current for the heater and take time to warm up - that's why in higher precision gear there's a warm up spec.

Not normally used in your average microprocessor based system! Such is the effect of temperature drift that the Maxim oscillators temperature compensate rather than try to keep a constant temperature.