ITunes

iTunes is the default music player in Mac OS X and most people generally use it. There is a windows version available but it is very heavy. It is very linked in with it's music store and iPods.

iTunes plays the most popular formats including mp3, apple lossless and many others. For files it can't play it'll automatically convert them. For digital downloads most people use the internal store but these have DRM (Digitial Rights Management) on them. If you don't have an iPod it'll be best to download from alternate stores like Tesco and 7digital