Tip of the cap to nats2playoffs, who used to compile sites that was the basis of this thread. quotes from him are below, with updates.
If you want to post a Game Day Thread (GDT), you ought to, at the minimum, post the official lineups for both teams.
Unfortunately, the best place N2P recommended, the baseballpress site, went kaput in the middle of 2023.
Many of the fantasy or betting sites have lineups, but they may be blocked at work. For example, N2P recommended rotowire:
Rotowire lists them. In 2019 they display "Confirmed Lineup" and "Expected Lineup" (an UNRELIABLE GUESS):
https://www.rotowire.com/baseball/daily-lineups.php
MLB now has a rule requiring when teams must post lineups, to try to preclude fraud in online betting. MLB is posting an UNRELIABLE GUESS in 2019 here, without telling anyone that the lineup has NOT been submitted yet:
https://www.lineups.com/mlb/lineups
My impression is that MLB Gameday is pretty good now and of course Fangraphs posts lineups.
FG is under the "Live Scoreboard" tab under their "Scores" tab on the home page.
Other recommended sites, please post below.
Note - After the Nats win, go to the first post and hit the "modify" button. The drop down menu right underneath the thread title is "message icon." That gives you a choice of ways to mark your thread in the index. At the bottom is the choice of "Curly W." select that and then hit save.
