A few weeks ago, I launched a new utility called DrupalVersion.com. It has one simple purpose: find out if your sites are up to date with the latest version of Drupal (6 or 7). It was a weekend project that provided a solution to a need we have at Thrive, to find out if any of our retainer clients' sites need updating, all in one place.
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DrupalVersion.com is an easy way to check Drupal versions for multiple sites at one time.