Check out the files and the release notes associated with each version at the SourceForge XAMPP File Repository
Click on a version then look for each versions release notes by clicking on the small round black i button just before the time line link and then click on the Release Notes link below the MD5 check sums.
For example here is the link to the version 1.6.6 release notes:http://sourceforge.net/project/shownote ... _id=573790
Here is the release notes for version 1.5.0:http://sourceforge.net/project/shownote ... _id=368900
You will find the XAMPP component versions in the release notes - slow method but it is the best available, see below about a list that used to be available.
There used to be a German Wiki that had a list of each versions contents but like all good things it seems to have disappeared in to thin air - if anyone still has a link or knows of a change of address then please post it here as the list saved much time and energy searching for each versions contents.
This was the old address that now gives a 404 not found error:http://faq.kwm-web.info/doku.php/xampp_versionen_und_inhalt:windows