sqlplus /nolog

This allows you to start a SQL*Plus session without connecting to a database:

TEST11>sqlplus /nolog

SQL*Plus: Release 11.1.0.6.0 - Production on Mon Feb 7 14:01:41 2011

Copyright (c) 1982, 2007, Oracle.  All rights reserved.

SQL> SELECT SYS_CONTEXT('USERENV','DB_NAME') DATABASE

  2  FROM DUAL;

SP2-0640: Not connected

SQL> SHOW USER

USER is ""

SQL>