What do you do with prisoners?

In one of my games I noticed that I have almost 20 prisoners and about 5 of them had been in my dungeons for more than 20 years. On poor woman have been prisoned more than 40 years (I guess it was during some war)! Many times when I try to ransom them I get negative response and that is how they stay there. I can't execute them because of the penalties. I did found it to be a very good way to keep couple of dukes from getting children (since their wives were prisoners of mine). This trick does not work against the Muslims though.

So, what do you do with them and what is the longest someone have been your prisoner?