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.


Exceptions:

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'.