File extension webm is used by following file type(s):

File extension webm file type description:
Default file extension for .webm file type:
Matroska WebM video file

The WEBM file suffix represents WebM video files. WebM is an open, royalty-free, media file format designed for the web. WebM defines the file container structure, video and audio formats. WebM files consist of video streams compressed with the VP8 video codec and audio streams compressed with the Vorbis audio codec. The WebM file structure is based on the Matroska container.

that open webm files:
Arista Transcoder (Linux)
FFmpeg (multiplatform software)
Firefox (Microsoft Windows)
Firefox for Linux (Linux)
Firefox for Mac (Apple OS X)
Free Studio (Microsoft Windows)
Google Chrome (Microsoft Windows)
Google Chrome for Linux (Linux)
Google Chrome for Mac (Apple OS X)
MediaCoder (Microsoft Windows)
Miro Video Converter (Microsoft Windows)
Miro Video Converter for Mac (Apple OS X)
MPlayer (Microsoft Windows)
MPlayer for Linux (Linux)
MPlayer for Mac (Apple OS X)
Opera browser (Microsoft Windows)
Opera browser for Linux (Linux)
Opera browser for Mac (Apple OS X)
Safari (Apple OS X)
Safari for Microsoft Windows (Microsoft Windows)
Squeeze (multiplatform software)
VLC media player (Microsoft Windows)
VLC media player for Linux (Linux)
VLC media player for Mac (Apple OS X)
Winamp (Microsoft Windows)
XMedia Recode (Microsoft Windows)