Google's Flights search has generally been panned by the frequent flyer community for being too focused on commodity flights where price and shortest duration trump all else. However, it's blazing fast speed makes it a great tool to start your mileage run planning.
There are some some good tools hidden behind the seemingly featureless interface. First is the stock standard searching. Start by specifying your departure and destination. Immediately (and I mean immediately!) Google Flights presents flight options and pricing for a default set of dates. You can then change the departure and arrival dates below the map to your specific travel plans.
The data is sorted by time and price and defaults to showing "Efficient Flights", hiding longer and more expensive flights.
You can then filter your results to show all results, or use the Limits icon to drag the price and duration slider to limit the number of flights shown.
While this is helpful for the average flyer, the more interesting tools are found by changing the calendar and destination fields. First, you can limit your search to your preferred alliance: OneWorld, SkyTeam, or Star Alliance, or a specific airline.
The calendar icon (next to the return date field) isn't simply a date picker that only pops up a dumb calendar, but instead this calendar is alive with near real time pricing updates. Use the slider to adjust your dates or hover over the bars looking for the shortest bar indicating a lower price on another date. (Since most days were the same in my example below, I highlighted a more expensive trip the week before to show the difference.)
Lastly, leave your destination field blank and see what destinations you can fly to. A great way to start a mileage run planning session before heading to ITA to build a multi-segment run.
While not feature complete, with the recent addition of international destinations this tool is quickly becoming more powerful. I'll be watching to see what features they release next. Hopefully they'll include some of ITA's routing language directly into the product to have the benefits of speed and flexibility in crafting routes for mileage runs.