Don't get me wrong... I am in full support of just wandering out into the unknown with a camera, but when the stakes for a photo are high, sometimes a little research beforehand is a good idea. For a prime example of the results of great planning, check out Salomon Freekski's Eclipse.
Here are five free tools I used for scouting during my travel series from Namibia. They allowed me some visualization of ideas and improved their probability of coming to life.
This online app gives information about sunrise and sunset, dusk and dawn duration, and plots a line of the sun's position for any time during the day. It's been my go-to since around 2010 and has been improving since it's inception.
2.) The Photographer's Ephemeris
Basically a supercharged version of Suncalc. It adds the functionality of the sun's elevation and azimuth, moonrise and moonset, and also comes in the form of an iOS and Android app! It's especially useful if you are trying to visualize how the sun will be blocked by certain landmarks, or looking for interesting shadows.
3.) Google Earth / Maps
It would be very remiss of me NOT to mention some sort of Google, seeing as how I integrate some form of their expansive tools into many facets of my workflow. The ability to look at a 3-D model of the earth and get an idea of landscape are of paramount importance, and can immediately visualize your idea. Here is the same Namibian landscape visualized through Google:
4.) Darksite Finder
This website uses historical data to generate a map of the world's light pollution. I can't comment on the dataset it queries since the only mention is "2006 data", but at minimum it should give a general idea of how dark of skies to expect. I expect that it's resolution within the US and Europe is better than Africa.
On the flipside of the darksky tool is Skyfire - a bundled app with The Photographer's Ephemeris that gives forecasts for the quality of sunset. Check out their FAQ for a detailed description of the research that went into their tool.
Extra - 500px Places
I suppose this is more of a bucket list type of collection rather than site-specific scouting, but there;s a whole world of travel lust awaiting at 500px...
I hope this post was helpful! If anyone has any additions to the available resources please drop me a line!