sndpeek - a real-time audio visualization tool (animated, 3D)
Digital Level Meter - Display sound level
Youlean Loudness Meter - Find your audio's true perceived loudness to prepare for TV and streaming services.
AudioTool - Whether you want to make music for the club, or you're aiming to put a few tracks to a film, Audiotool is a website geared towards putting the mixes at your fingertips. Read more about AudioTool.
Sound Level Meter - You can measure the environmental noise in decibels.
Spectro - Spectro is a freeware audio file analyzer for windows.