• FLAC
A popular open-source lossless audio format, playable in many open-source programs.
• ALAC
Apple's lossless audio format, playable in iTunes. Has the extension .m4a like AAC files and is only distinguishable using another program.
• WMA Lossless
Microsoft's lossless audio format, playable in Windows Media Player. Has the extension .wma like normal WMA files and is only distinguishable using another program.