Similar in meaning:
* Modest
* Unassuming
* Down-to-earth
* Self-effacing
* Lowly
* Meek
* Submissive (can be negative)
* Unpretentious
* Simple
* Plain
More specific meanings:
* Deferential: Showing respect and consideration for others
* Respectful: Showing honor and esteem
* Subservient: Willing to obey and serve others
* Contrite: Feeling remorseful for one's actions
* Penitent: Showing regret for wrongdoing
Negative connotations:
* Cringing
* Submissive
* Cowardly
The best word to use will depend on the context of your writing or speech.