If you just need a quick way to convert a music file to MP3 format, or a Word Document to PDF, or a BMP image to a JPG file, you can do it online for free with Zamzar. You may have to deal with a pop-up ad or two, and there is a limit of 100 MB for the free version, but that’s plenty of space for almost any common audio file, document file, or image file. It’s probably not the best free tool for video conversion because of the limit, but it looks like it would be convenient in some situations.