This version focused on fixing numerous bugs within the Drupal core. According to the release announcement and changelog, no security issues were patched and no major new features were added this time around other than some core API additions.
The only notable addition is are two new functions that allows developers to specify (and retrieve) an arbitrary active menu trail, functionality that existed in Drupal 5 but which vanished after an API reworking in Drupal 6 caused a functionality regression. After more than two years of discussion and workarounds, one of the oldest active bug reports for the Drupal core has finally been closed.
Other notable bug fixes include a correction to OpenID authentication that did not properly adhere to the spec, significant improvements to the way the information is presented after the "clean URLs" test returns a fail result and fixes to a case where uploaded files would vanish.
Grab Drupal 7.9 from the project page here.