Audacity is an open-source and cross-platform (Windows, Mac OS X, Linux) live audio recording and editing freeware. It is developed under Linux, has a point-and-shoot operation interface and professional audio processing functions and effects.
Ultrastar Deluxe is a free and open-source karaoke entertainment application for PC users, its inspiration comes from the SingStar music video game provided by SONY PlayStation console. It allows up to six players to sing along to a song or music video, and let players score points according to the pitch of voice and the rhythm of singing.
TinyPiano.c is a simple yet satisfying piano emulator (turns your PC keyboard into an electronic piano), gives 15 tones max, with 120+ different sound effects, and sounds great. Besides, it is so tiny that even smaller than a blank word document.