For over a couple of years now, Google has been leveraging data from Google Maps' install base to deliver these sorts of insights.
You can also click an hour other than the current one to see Google's guess for that time of day, if you're trying to plan ahead.
Google already uses this data to show you when a restaurant will get busy through the "Popular Times" section on each restaurant listing. Now though, in a development we can't believe didn't happen sooner, Google will show you the wait times of almost a million sit-down restaurants around the world. To calculate wait times, Google analyzes patterns of customer visits over the last several weeks. This could come in particularly handy for those bars and restaurants that don't take reservations (looking at you, Monk's). Not only will it tell you which times are most popular among visitors and your estimated wait time for a table, but it will also tell you how long people typically spend at the business you're visiting.
Google premiered a new feature Tuesday that could save you from standing in line for groceries or being shunned by the host at your favorite restaurant.