Drupal 7.15 Released
Drupal 7.15 released on August 1, 2012.
Generally, there is no major change in Drupal core 7.15 in comparison to Drupal core 7.14. There are mostly bug fixes (no security bug fix). According to Drupal.org, no major feature is added in this release, but it is strongly recommended for Drupal 7 sites to upgrade to this version .
You can find the full list of Drupal 7.15 changes on Drupal 7.15 page.
If you are using Speedy module, I believe you must first disable this module, and then go through updating process. I had Speedy module installed, and tried to update 7.14 to 7.15, updating failed.
You can always the latest version of Drupal core from Drupal project page.