I've tried a few of these. Mapmyrun.com was at one time the leader (and may still be). Mapmyrun has a sister site, Mapmyride.com. Actually, I have no idea why they differ, though presumably they direct different advertisements to the user.
They all use the same software based on Google maps. Refinements the better sites offer include following twists and turns between points rather than drawing straight lines.
I now use www.runningmap.com exclusively. Mapmyrun is either only available with a Facebook account, or it is trying hard enough to push me there that I'm not willing to find the work-around. Others at least want me to sign in so they can direct running shoe advertisements to my e-mail inbox. Runningmap.com just gives the interface, permits me to select a starting point by relatively anonymous information like a zip code or street crossing, and permits me to save routes (giving me a link), and asks for no information or commitment. I have a quaint habit of starting and stopping all of my runs a few blocks away to preserve some feeling of privacy (though I have little question my runs are easily traceable to my front door by a sufficiently savvy investigator).
I've now formed the habit of mapping hypothetical runs while watching TV. "What if I went further on Beach drive instead of turning into Bethesda at Cedar Lane?" "What if I took the Monon Trail all the way to 70th before heading west to the canal tow-path?" It's a less expensive habit than shopping for running shoes as a way to get my fix in the quiet times.