IconVault's Image Noise adds film grain to any photo with fine control: an intensity slider, a choice between monochrome and color noise, and a seeded random generator, so the exact same grain pattern can be reproduced whenever you need it.

Grain is a classic way to add texture, hide banding in gradients, or give digital photos an analog film look. Everything runs in your browser with no upload and no signup, and it is free. Honest note: noise is baked into the exported pixels; keep the original if you might want a clean version later.