Official homepage: X Lossless Decoder
If you are an iPod owner and value high quality audio then you surely use lossless formats. iTunes however can only use the Apple Lossless format and if you have other formats lying around like flac you need to decode the tracks. A very easy to use, fast tool for that is X Lossless Decoder by a japanese developer. It is freeware and can handle various lossless formats.