A rhythm game based heavily off of Guitar Hero and Rockband. It plays similarly to modded Guitar Hero 3, but with a slightly tighter note hit window. It's free, but not open source (yet?) and is available for both Windows and Linux. They don't have a website yet, but are working on getting one made.
Alter/Ego is a free vocal synthesis VST. Some of the voicebanks available for it are free, but some are paid. It's not very high quality, but for something that's free, it's not too bad. As of my time of writing, there are voices available in English (free and paid), Japanese (free and paid) and French (paid). Alter/Ego is available for Windows and macOS.