There are at least two cross-platform projects where the biggest expense is a microphone instead of software.
1. My project, Talon. Windows/Linux/Mac support, and a first party local speech recognition engine that is pretty good and getting better. It’s free, but the engine is in a private beta (which is $15/mo to support development, optional if there’s a financial issue).
2. Serenade. They are VC backed. Currently free, unsure about their longer term plans. They use cloud based recognition.