Do you want to use a free ringtone for your mobile (cell) phone? Free as in freedom? Here are some links to ringtones published under a free license, e.g. Creative Commons, GPL. Not much yet but I hope it works as a start. You may also find links to Linux tools to generate and convert ringtones. If you know of a missing ringtone or tool please let me know.
Ringtone Tools(http://nakentone.naken.cc/tools.php) is a set of tools for creating ringtones on mobile phones. rtttl is a popular format for ringtones, you may use it to beam a new ringtone via SMS to your phone. Never pay for ringtones anymore, get them for free.
RTTL2WAV(http://www.iki.fi/karij/software/) converts a RTTTL-formatted mobile phone ringing tone to a WAV file. The output of this program can also be copied to stdout as a raw PCM data (linear 44.1kHz signed 16bit integers). It also includes a shell script which allows you to listen ringing tones directly with your sound card.
Midi2C25(http://efsa.sourceforge.net/archive/leitner/midi2c25.htm) is a small Eiffel application that takes a standard MIDI file and gives you the intructions on how to get the same tune as a ringtone on a Siemens C25 cell phone. There are two versions: A GTK GUI and a command line interface.
This is a simple Perl script to convert between AMR(http://www.xa.bi/mms/) and different audio formats (WAV, MIDI & MP3).
Media-Convert(http://media-convert.com/convert/index.php) is a free online service that can be used to turn ANY audio file into just about ANY ringtone format - and it's all online so it's OS independent.