The new Audit Log's Marketplace Login Activity page lists when a user successfully logs in to Google Apps Marketplace applications installed in your account.

Each auto log entry describes a user's successful Marketplace login activity including the Marketplace application's name and login page URL, the user's primary email address, and the activity's date.

Editions included:
Google Apps, Google Apps for Business, Government and Education

Languages included:
All languages supported

For more information:
http://support.google.com/a/bin/answer.py?hl=en&answer=2498547

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