Myxer, a site offering free, advertising-supported mobile content, has added the ability to download iPhone-formatted ringtones to its service. To access the ringtones, users can visit myxer.com, browse the available content, and use the “Download for iPhone” link to download any tone to their PC or Mac desktop. The iPhone ringtones are already in the required .m4r format, so they can be added straight to iTunes following the download,…