Accelerando is a musical metronome that can speed up, allowing you to practice your music at progressively faster tempos. For example, you could set it to play 60 beats per minute for 4 bars, then automatically speed up by 10 beats per minute, and so on.
It runs on Unix. It has dependencies on Perl/Tk, sox, and festival, all of which are available via Debian apt-get. To install it, download and unpack the tarball, and do a "make install".