General:
* Whisper
* Murmur
* Mutter
* Speak softly
* Communicate in hushed tones
More specific:
* Confide (in a whisper) - To share a secret
* Coax (in a whisper) - To gently persuade
* Counsel (in hushed tones) - To offer advice
* Converse sotto voce - To talk quietly, often in a secret or conspiratorial way
Informal:
* Hiss - To whisper harshly
* Chirp - To speak softly and quickly
* Snuggle - To talk in a cozy and intimate way
The best word to use will depend on the specific context and the tone you want to achieve.