This site has overviews of popular audio and video formats and list of free software which can convert these formats one into another.
What is MP3 file
It would be good to introduce you into what stands behind MP3. If you're not interested in details of MP3 format and just want to get free "MP3 to something else" converter please proceed here.
MP3 file allows storing your audio using lossy compression. What lossy means is that not all details of audio stream will be stored. There is usually a tradeoff between audio quality and number of megabytes it will take on hard drive. This tradeoff is introduced using "bit rate" which you can define when creating MP3 file. Bit rate defines how many bits are going to be used to store 1 second of audio. For example at bit rate of 128kbit/s a typical CD track converted to MP3 would take 10 times less disk space.
Note that even though quality of MP3 file is great it is still below typical CD track quality. Audiophiles thus favor lossless compression.
But if you're not an audiophile and if CD track and 128 kbit/s MP3 file sound exactly the same then you should go with MP3 files since it will save you tons of space.
Popular Audio Formats
Most common audio file formats are:
And here is a list of popular video formats:
Popular Video Formats
We tried to describe only most popular media file formats. For complete details on various file extensions visit File Extension Database.