If I recall correctly the user "postgres" has no database password set on Ubuntu by default. This implies, that you could log in to that account simply by using the "postgres" OS user account.
Hoping, that you have root access on the box you can do:
sudo -u postgres psql
If this fails with a database "postgres" does not exists error, then you are most likely not running on a Ubuntu or Debian server. In this case, just add template1 to the command:
sudo -u postgres psql template1
If any of those commands fail with an error psql: FATAL: password authentication failed for user "postgres" then examine the file /etc/postgresql/8.4/main/pg_hba.conf: There must be a line like this as the first non-comment line:
local all postgres ident
For the newer versions of the PostgreSQL ident actually might be a peer. That's OK also.
Then Inside the psql shell, you can give the database user "postgres" a password:
ALTER USER postgres PASSWORD 'newPassword';
You can leave the psql shell by pressing Ctrl+D or with the command \q.
Now you could give pgAdmin a valid password for the DB superuser.
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