After single tone monophonic ringtones, polyphonic rintones where introduced. Those ringtones support multiple tones being played simultaneously using different sound samples like piano or drum sounds. Using different "channels" at the same time, a more natural sound than monophonic sounds alone are possible.
Polyphonic ringtones are created and saved as a MIDI (Musical Instrument Digital Interface) file. While lots of ringtones are downloadable for free, actually every MIDI file (if it is compatible with your cell phone features, ie. number of used channels are not exceeded etc.) can be used as ringtone. So all you need to get polyphonic ringtones is to search for MIDI files on the net or you play and save a song yourself with an editor) and get those songs onto your phone.
How do you get those files onto your mobile? If you want to get ringtones from websites, there are usually two options. First, you could choose a song, enter your phone number and get a SMS providing a link for a WAP connection (additional connection costs) or you directly get a WAP address you can connect to with your phone (needs to support WAP though). On the other hand, you could easily download your song (or wallpaper,etc.) and add it to your phone manually. Then, your mobile (and your PC) has to support some kind of the following transfering possibilities: bluetooth, infrared or cable (e.g. USB).