January 10, 2020 (updated August 2020):
A number of other places on the web are devoted to Washington, D.C., baseball, past and present. Here’s are links to some of them:
“The Expansion Nats” Facebook site:
https://www.facebook.com/groups/463338547208950/ . Because the page is now private, you need an invitation from a current member to join.
D.C. Baseball History Yesterday and Today:
and the associated Facebook page, Washington D.C. Baseball – Yesterday & Today:
https://www.facebook.com/groups/60009109815/
A Facebook site devoted to Frank “Hondo” Howard:
https://www.facebook.com/groups/293945135091/
Squibber is the quarterly newsletter of the Bob Davids Chapter of the Society for American Baseball Research. Join SABR and the chapter to read interesting articles about D.C. baseball.
Suggestions for additions are welcome. Of course, the team’s official web site, washingtonnationals.com, has many useful features, including all-time leaders lists for the Expos/Nationals franchise.
(I’m posting the Facebook links, even though I am no fan of Facebook and have tried to fully delete my own Facebook account.)
As far as I can tell, D.C. Baseball 
History has a strong connection to the Washington Baseball Historical Society. I’m not sure the society is still active as it once was. I know that Nats News, the quarterly newsletter of the society, is no longer published and its longtime editor, James R. Hartley, died in August 2020. The web site and Facebook page, run by Mark Hornbaker, seem to be going strong.)
