As a runner who also travels quite a bit, I like to explore new towns while running. I use online maps to plan my runs or to find out how far I ran after I get back. It depends on what I plan to do with the map as to which online map tool I choose. Some seem easier to use for certain purposes, though many of them have similar features.
Most online maps for runners use Google Maps as their base. This is nice because it gives a similar feel to the mapping tools. Online maps that use other map software aren't any harder to use, just slightly different. If they don't use Google maps they usually use either Microsoft Maps or OpenStreetMap.
Online Maps: RunningAHEAD
For the purposes of just knowing how far I ran, or for planning out a one-time route, I use the mapping program inside of RunningAHEAD. This is the website I use for my running log. The mapping program is somewhat simplistic, but since I am always logged into their system, it is the one I most often choose.
With the mapping program inside of RunningAHEAD, which uses Google Maps as its base, you can measure and save your common running routes. This allows RunningAHEAD members to save different runs that they can choose from a list in their running log. This keeps them from having to manually type in the distance of the run each time.
The tool will follow roads making route creation easier or it is easy to switch to creating an off-road route. There is also an elevation profile that shows for the maps. You don't have to be a member to use the mapping tool, but if you are not saving the maps into your training log, this program has no compelling reason to use it. It is possible to share maps with friends, but not easy.
Online Maps: MapMyRun
MapMyRun is the most full-featured mapping program I use. This is the website I would recommend if you want to annotate a map. This feature is helpful when you are coordinating a run with friends and want to show where the mile markers are and where water breaks are planned. After the run, the ability to annotate a map lets you put in information such as where Bob got distracted ran into the back of a parked car so you can all laugh about it later.
Because the routes you create can be saved and shared publicly or privately, it is the site I use when I want to share the route of a run with my friends back home. I recently ran in downtown Buenos Aires and created a map at MapMyRun to share with my parents back in the US. I was able to put notes in the map to tell them what I saw at various locations.
For race organizers planning water stops, bathroom locations and start and finish line details, MapMyRun is a good choice.
MapMyRun has a running log where you can keep track of your maps and run details. You can also browse through other people's public maps and find running routes in your area.
There is one thing that MapMyRun does not do well: simplicity. If your goal is to quickly measure a route and have no plans to save it, then this may not be the best tool. Also, if you are only interested in saving the route, with no further information, then there are other tools that do this easier. When saving a running route at MapMyRun you will be asked to fill in lots of extra information about the route such as whether the roads are paved, the environment is urban or if the course is hilly. You don't have to add any of that information, but if you are not planning to add the info, then using another tool that is simpler and quicker may be a better choice.
Online Maps: Gmap Pedometer
For fast and simple map creation that you want to share, Gmap Pedometer is your best choice. You are able to map the route of your run and get a URL that you can share with your friends and family. There is not much other than that you can do with Gmap Pedometer. For its simplicity, it cannot be beat.
This is an excellent tool if you need to share the route of a run without any concern for extra features. Once the map route has been created you can hit the "Save route" link in the left sidebar and you are given a URL you can email to your friends or family. I use this feature when I am running in a new town, or a route that I don't normally run, and I want my wife to know where I plan to be for the run. Thankfully, she has never had to come find me on a run, but giving her my planned running route in advance is reassuring to her. I usually only do this on a long run or when we are in a new town as all my normal daily runs are near the house.
One of the limitations to Gmap Pedometer is that you can't map an off-road run. The mapping program follows the roads when drawing its lines. Most of the time this is a feature you want in a mapping program, but if you are trying to measure a run through the park or woods, Gmap Pedometer will not be the right tool.
Even as simple as Gmap Pedometer is, there is still an elevation profile option in the maps. It uses Google Maps natively, but has an option to use OpenStreetMap. This feature allows you to submit changes to streets. If you live in an area that does not have Google Maps available (some foreign countries), then you can work with the OpenStreetMap team to create maps for the routes you run. Then your changes and additions will be available for other users of Gmap Pedometer to use.
Online Maps: Google Earth
When I first started using online mapping software for figuring out my running routes, I used Google Earth. I lived in a country that did not have Google Maps available. There are now maps for the city I lived in and Google is adding more all the time. But Google Earth has satellite maps available for the entire world.
Google Earth is still my favorite place to measure running routes when I am measuring off-road runs. The measurement tool is easy to use, even if a bit confusing the first time you use it. When looking for the tool, it is called the Ruler. The one thing that Google Earth does not do that all the other sites do is automatically follow the roads. While it is easy to measure off-road runs, it is more work to follow roads since nothing is done automatically.
Google Earth is probably the most difficult of all the choices to share your map with others. Someone wanting to view your map on their computer must have Google Earth installed. Even though it uses online maps, there is an installed program that interfaces with those maps.
Online Maps: Summary
If I could only choose one tool, I would stick with RunningAHEAD mapping tool for the simplicity of using it, the maps integrate well with my running log and I can save my running routes to share with others. MapMyRun would probably be my second choice since it does have every feature a runner would need.
Warning: Don't publicly share routes that you run regularly. If you include information like "I do this run every Thursday at 7:00 PM" on a publicly saved map, you could unknowingly invite trouble. It is also a not a good idea to publicly share routes that start and end at your house.
Try these tools out for yourself. They are the ones I use on a regular basis. There may be others that you like better. Please leave a comment and let me know. You could also sign up with InfoBarrel and write an article on what your favorite online maps for runners are.