Most Technologic Systems products:
The common default Linux account username is root.
These normally require no password for the root account. This should be set at the downstream developer's earliest convenience.
Ubuntu: Ubuntu has two users. Root has no password. The user "ubuntu" has a password "ubuntu". The default "ubuntu" user is in the sudoers group.
TS-3000 and TS-5000 series SBCs running Linux:
The default user is 'root' and the password is 'redhat'.