opus

winLAME is an easy to use encoder for many audio formats, including MP3, Opus, Ogg Vorbis and more.

FFmpeg Batch AV Converter is a free universal audio and video encoder.

Internet Jam Session Software

fre:ac is a free audio converter and CD ripper with support for various popular formats and encoders. It converts freely between MP3, M4A/AAC, FLAC, WMA, Opus, Ogg Vorbis, Speex, Monkey's Audio (APE), WavPack, WAV and other formats.