I for once wanted a custom ring tone on my phone. I have the Android G1 phone, so it's pretty easy, but not obvious

• Connect your phone to your computer, mount the USB (on the android it shows up in the notification area to mount)
• Explore the new mounted drive and notice there is no folder for ringtones.
  • Create a folder called 'ringtones'
  • Drop in any mp3 there
•  Now unmount the USB and edit your Android preferences
  •  Menu -> Settings -> Sound & Display -> Phone ringtone; your newly added mp3 shows up in the list now.