If you like to buy high quality music files, as many sites now offer files in lossless WAV format, you will probably need to convert them for use on a portable device.
Free Audio Converter is a tool that does exactly that. It can convert between MP3, M4A, AAC, WAV WMA and OGG files, unfortunately not the lossless FLAC format. However, that covers most formats you might see, and it almost couln't be easier to use.
Simply drag in any files you want to convert, choose a format, destination and quality for conversion, and hit Convert. It's pretty fast, and does it's job well. More advanced users might want more configuration options than are available, but it will cover most people's needs.
The only bad things about Free Audio Converter are that installation automatically opens your browser and loads the developer's homepage, And the buttons in the application's toolbar are largely useless.
Free Audio Converter is exactly what is says it is, and it's a reliable and relatively flexible tool for conversion of audio.