Free converter will convert lots of kinds of files - image, audio, video, ebook, etc. - into other kinds of files. Looks great but haven't tried it yet. Reply to this if you've used it!
"ImgOps is a meta-tool that lets you quickly apply many online image utilities to an image. " enter the url of an image and you can edit, add effects, convert format and other stuff. You can also add their bookmarklet or just insert http://imgops.com/ in front of an image's url