Is there a Drupal tool to aggregate and compress CSS files
I'm not 100% sure I'm right in thinking this, but if you're looking for something to reverse the CSS aggregation/compression, but if you are then it doesn't look like there is anything that exists that will give you exactly what was initially compressed. However, the cache Drupal creates is an array that is keyed by the original filename of the cached CSS, so you should be able to find out WHICH CSS files are being cached relatively easily (I think the variable_get('drupal_css_cache_files'); will get the array for you.

If you are looking to mimic what Drupal does when it aggregates and caches your CSS, then take a look at the drupal_build_css_cache() function (which, as an FYI, is called by drupal_aggregate_css()).

As a point of interest, there is also a drupal_build_js_cache as you'd expect.