LameXP is an easy-to-use graphical front end for the popular MP3 encoder LAME, which simplifies the process of converting audio files from one format to another.
Despite the name, it doesn't just run on XP, and it doesn't only support MP3 files. The program can input AC3, MP4/ AAC, ALAC, AIFF, DTS, FLAC, ADPCM, APE, MP1/ MP2/ MP3, Musepack, Shorten, Speex, Au, TTA, WAV/RIFF, WavPack and WMA files (the latter requires a separate free download), and it's able to convert these to MP3, Ogg, AC3, FLAC or WAV/RIFF files, and AAC if you download a separate component.
Don't be put off by the possible need to download separate components: LameXP really couldn't make this any easier to set up. If you don't have the necessary files to support the AAC format, for instance, it will warn you, provide the URL where you can download these, tell you which files it needs and the folder where they must be placed - it's very straightforward.
Encoding is generally just as easy. Drag and drop your chosen audio files onto the program, select your preferred output format under the Compression tab, click Encode Now!, then watch as LameXP converts your music. (Multi-threading support allows the program to convert several files at once, so this is very quick.)
If you need more control, though, there are plenty of advanced options just a click or two away. You get to choose the encoding method (VBR, ABR, CBR) and quality, sampling rates or normalisation options; a Tone Adjustment setting lets you consistently tweak the bass and treble for all your input files; you can enter metadata that will automatically be added to the file, replacing whatever was in the source; and if you really know what you're doing then you can add custom parameters for each of the encoders used by LameXP.