Jetpack introduced a new Activity feature that monitors your site and adds a new tab in WordPress.com where you can see all your site’s activity outlined in an “organized, readable way.” How far back you can look or how many features you get (such as filtering) depends on your plan.
You’ll find similar plugins in the repo (here’s one and here’s another), and there is a nice comparison from John Hughes at Themeisle that considers the major activity log plugins as of last year.
