CT MIK is what you need.
Its wicked and has become a lot faster and more accurate with recent updates. Well worth the small purchase fee.
Anyway MIK will scan your tracks and detect the bpm and key for each, you can then have these written to either the ID3 'comment' field or the the 'key' and 'bpm' fields.
If you are using Traktor the key will show up in the file browser and most other mixing software I think.
One thing to note though is that if you are using Traktor you
do not want MIK writing to the BPM field as Traktor does not seem to like this. Instead let Traktor find the BPM's its self while analysing the waveform etc. Duno if you are using Traktor but its important info for anyone that might be.
Oh and the MP3 Tag program to end all which will let you view them explorer style...
http://www.mp3tag.de/en/
Here's an example of what I think your after...
http://www.audiogenus.net/misc/tags.jpg