General:
* Humble
* Unpretentious
* Down-to-earth
* Modest
* Unassuming
* Simple
* Genuine
Specific to social status:
* Egalitarian
* Democratist
* Equalitarian
* Commoner (in the sense of being one of the people)
Related to attitude:
* Open-minded
* Welcoming
* Tolerant
* Accepting
* Friendly
The best antonym will depend on the specific context in which "snob" is being used.