Need a quick ringtone or an isolated snippet of audio from a bigger file? CutMP3.Net does it with a file upload button, two sliders and a "Cut" button.
But when you're away from your own laptop or looking to make a simple extraction from a song or audio file, CutMP3.net lets you upload, play and use sliders or precise time markers to select the boundaries of your portion. Hit "Cut" and you're prompted to save your new snippet. Depending on your phone, it's then pretty easy to email it to yourself or save it to memory to make a ringtone, if that's what you're after.
CutMP3.net is free to use and requires no sign-up. There's no file size limit listed, but since you're doing the uploading yourself, it's better used as a small file utility.