General synonyms:
* Quiet
* Hush
* Still
* Mute
* Soundless
* Noiseless
* Unspoken
* Unvoiced
More specific synonyms:
* For people: Speechless, mum, tight-lipped, taciturn, reticent, withdrawn, reserved
* For places: Serene, peaceful, tranquil, hushed, secluded
* For objects: Mute, inanimate, motionless
Figurative synonyms:
* For emotions: Suppressed, bottled up, unexpressed
* For agreements: Tacit, implied, understood
The best synonym for "silent" will depend on the specific situation and the meaning you want to convey.