General Synonyms:
* Adrenaline junkie
* Risk-taker
* Thrill-seeker
* Stuntman/stuntwoman
* Adventurer
* Reckless (often with negative connotations)
* Audacious
More Specific Synonyms:
* Skydiver
* Base jumper
* Acrobat
* High-wire walker
The best synonym will depend on the specific context and the intended meaning.