The DNA-Newbie mailing list is where I turned for help when I first got into genetic genealogy five years ago. I met awesome people like CeCe Moore who answered my questions, pointed me to books and other resources, and helped me understand what I was looking at with all these test results.
The Alsace-Lorraine mailing list helped me find one of the most amazing websites out there, with literally hundreds of years of documents of my German/French ancestors from this part of Europe. Below are some of the ancestors I was able to find with these records.
The German Bohemia mailing list has some of the nicest, most helpful people on the planet. They answered my questions, dug into social and political history to explain obscure words, and helped me decipher the difficult German script, an example of which is below.
The Quebec genealogy mailing list helped me identify where in Quebec my French-Canadians were from, and helped me learn to navigate the French-Canadian records on Ancestry and other sites. Their help was crucial in solving a DNA problem I had with my French-Canadian ancestors, and finding the info would have taken much, much more time without their help.
The German-Volhynia-Poland mailing list helped me find records of my Germans in Russia, which led to one of the biggest brick wall busts in my genealogy career so far. I never would have guessed to look in church records written in Polish, kept by the Russian government, to look for my German ancestors without their expertise.
I'm sure there are others, but those are the ones that come to mind right away. I am a firm believer in the power of mailing lists, so if you are stuck on a line or an ancestor, find a mailing list that covers your topic. There may be someone with the answers you need just waiting to help!