General:
* Understanding
* Forgiving
* Patient
* Open-minded
* Broad-minded
* Liberal
* Accommodating
* Flexible
* Permissive
* Accepting
More specific:
* Enduring (for physical situations)
* Resistant (for biological situations)
* Immune (for biological situations)
* Indulgent (for behavior)
The best synonym for "tolerant" will depend on the specific context in which you are using it. For example, if you are talking about someone's attitude towards different cultures, "open-minded" might be a good synonym. But if you are talking about a plant's ability to survive in harsh conditions, "enduring" might be a better choice.