If you are only using MP3s, there are may ways to alter/add metadata. For MP3s the metadata is called ID3 tags:
There are many different ways of altering the metadata in an MP3, iTunes is probably the easiest (if you use a mac and itunes as your library), there is also a program called Library Monkey which is much more versatile:
The other format that works well with metadata is Broadcast WAV (BWAV). Programs such as Soundminer work well with bwav files. Soundminer's metadata format is slightly different than ID3 but most of the same information is there and you can change and alter as needed. Also, many SFX libraries come already with Soundminer metadata added so all you need to do is "add to library" and you are rolling. Also, if you export WAV files from Pro Tools, the Timecode location is burned into the file as well (often required when exporting final mixes and stems for delivery)
If you are creating sound files to have in your own library, and you have the space available, I wouldn't compress down to MP3 just to use metadata, you can never get back to your full-rez audio after that.