Aria Maestosa is an open-source (GPL) midi tracker/editor. It lets you compose, edit and play midi files with a few clicks in a user-friendly interface offering keyboard, guitar, drum and controller views.

Software editor of many softwares, including Cabrio a free MP3 player.

Emagic US web site.

Makes virtual instruments including ManyStation, ManyGuitar, and ManyOne

AMNS makes Nightingale, a professional music notation software for Macintosh

Editor of MyMusic, this software lets you create your own music library. MyMusic is a powerful database program designed to help you maintain an inventory of your music collection. 

Editor of several M.A.S Plug-ins, including Vintasion and Fattenizer.

Software developer for the business of Music: Music Production Deluxe, Production Label Pro 2.1, On The Road...

NOTION - Music composition and performance software with the most intuitive composition interface and most realistic playback

Gerzonic, home of ambisonic vst plugins