Aaron Butacov

It adds a simple month-by-month calendar that shows all the months you have future posts for (and the current month no matter what), it highlights the days you have posts for, and as an added bonus if you click a day, the Post Timestamp boxes change to that day, month and year.
Also includes a widget so you can show users what upcoming posts you have on your website.
If you want to ad a widget to your dashboard, uncomment the line <code>add_action('wp_dashboard_setup', 'fpc_setup_dashboard_widget');</code> in the future_calendar.php file.

Future Posts Calendar

This plugin adds a monthly calendar that shows all the dates you have future posts.

Mark Jaquith

Some WordPress sites have problems with scheduled posts, where they don’t get published at the right time. Instead they appear as “missed schedule”. This plugin monitors scheduled posts, and makes sure that doesn’t happen.

Scheduled Post Guardian

Watches over scheduled posts, and makes sure they don't miss their scheduled time

Future Posts Calendar vs Scheduled Post Guardian