General:
* Agent
* Secret agent
* Infiltrator
* Undercover agent
* Informant
* Espionage agent
* Intelligence operative
* Covert operative
* Case officer
* Double agent (someone who spies for both sides)
* Sleeper agent (someone who stays dormant for a long time before activating)
* Mole (someone who has infiltrated an organization for spying purposes)
Informal/Slang:
* Snoop
* Snitch
* Rat
* Informant
* Scouting
* Private eye (for a detective)
* Shadow
Literary/Historical:
* Secret service
* Secret agent
* Intelligence officer
* Counterintelligence agent
* Clandestine operative
* Emissary (someone sent to carry out a secret mission)
Specific roles:
* Counterintelligence agent (someone who works to stop spies)
* Signals intelligence (someone who specializes in intercepting and analyzing communications)
* Human intelligence (someone who gathers information from people)
* Imagery intelligence (someone who analyzes satellite or aerial photos)
* Technical intelligence (someone who analyzes technology and weapons)
The best word to use will depend on the context and the specific nuance you want to convey.