Tune is an alsa-based, audio tuning application for linux. It can be used for tuning instruments or voices.
Tune has four viewing modes: Wave, Circle, FFT, and Notegraph. The wave view displays the actual sound wave coming into the default recording device. The circle chart will display which note is currently being played as well as the frequency of the ideal note. The FFT view displays the Fourier transform of the sound wave and marks the detected base note with an orange line. Finally, the notegraph will display notes that are played vs. time (similar to Karaoke Revolution). Notegraph is currently unimplemented in Tunecpp.
Here are some screenshots of tune 0.4.
Tune requires glibc, gtk+, alsa, and fftw-3 (for the fft of course). Once the appropriate libraries are installed, download the tune source code. Type make to build tunecpp or (if you want the old version) type make tune-c to make the c-version of tune.