Audiophiles love FLAC files for their lossless fidelity, while Ogg and WMA files aren't widely supported. With free plug-ins, all three of those formats can be played and managed in iTunes.
MacWorld links to and explains a number of free iTunes plug-ins, both official (Windows Media for QuickTime) and third-party, that make Windows Media Audio, freely-licensed Ogg and FLAC files compatible with iTunes libraries on Windows or Mac. In the case of FLAC files, you'll actually have to convert the file types from .flac to .mov, but the article explains the easy way to do that with Mac's Automator. On Windows systems, we'd suggest the previously mentioned Bulk Rename Utility. Found another way to make Apple's generally walled-off media manager play nice with non-native formats? Tell us how in the comments.
Play .wma, Ogg, and FLAC Files in ITunes [PC World/MacWorld]