A free app for Android, by MeiHillMan.

Ringtone Maker – MP3 Cutter is the best way to create custom ringtones from your favorite songs, save them as MP3, WAV, AAC, AMR, 3GPP, 3GP, and M4A files, and save them to your device. It is very easy to use, just select a song, choose an audio format, specify a clip size and start time, then save it. Ringtone Maker will automatically save the resulting MP3 file to your device. You can then use it as a ringtone or as a notification sound.

The application is very easy to use, and the interface is simple and clear. You can use the app to select your song, audio format, and clip size.