Informal:
* Rat
* Snitch
* Informant
* Fink
* Squealer
* Grass
* Narc (especially for police informants)
* Turncoat
* Judas
* Agent Provocateur
More colorful/humorous:
* Singing canary
* Chatty Cathy
* Whispering willow
* Loose cannon
* Loose lips sink ships
It's important to note that these terms are generally considered derogatory and should be used with caution.
Please remember: The best way to communicate is to use clear and respectful language.