Fre:ac is an open source audio conversion software with helpful features and professional level capabilities -- all available for free.
Good format selection: Along with the ubiquitous MP3 and WAV formats, Fre:ac also allows you to work with more obscure, but still useful formats, such as OGG, VORBIS, and FLAC.
CD ripping capabilities: Fre:ac also works very well as a CD ripper. It has access to an online database with all the information you need to automatically populate all the metadata attached to the files, so they can be sorted and organized properly in your media library.
Quality settings: Fre:ac offers professional-level control over output files. Instead of being locked into a preset or a one-size-fits-all output, you can specify exactly what bit rate and quality you want your files to render at.
Outdated interface: Fre:ac looks fairly outdated and is quite bare bones in appearance. Its interface is not terribly intuitive at first glance.
While Fre:ac's window dressing might be less elegant than some of its paid competitors, the power beneath the interface is as good or better as most other options.