Here's a good site to download pre-made ringtones:
http://www.polyphonicringtonez.com
Here are some free utilities for composing ring tone cell phone music:
Nok2Phone - Input MIDI files, NokRing text, or RTTTL text, and it will spit out Ringtone text files, Nokia MIDI files, Nokring text, MonkeyTone PDB files, Ericson E-Melody files, or iMelody format. This program comes from RingTone City. *This program supports many different phone brands(motorola,ericsson,samsung,etc.), even though it's name implies only Nokia.
Nokring - RTTTL composer. It will send the ringing tone to your phone if you have a special cable to do so.
Nokia 3210 RingTone Converter - Input RTTTL or Nokring text files and it will spit out Nokia keypresses, 3210 Composer files, RTTTL, or Nokring text files.
Mid2RTTL - Command line utility that converts MIDI files into RTTTL format. It uses notes from channel 1 by default, all other notes are ignored.
You sometimes need MIDI file format zero to load a ring tone tune onto your cell phone. Music MasterWorks only saves in format 1(multi-track format - MM will read format 0 files but not save in format 0), but never fear: Here is a utility that will convert format 1 to format 0:
Click here to download the MIDI file format converter, from format 1 to format 0
Here are the steps to use this utility (this is a DOS utility, so there is no easy GUI):