Connecting to an Oracle database on the command-line without using the Oracle client software?
If you download the free Oracle Instant Client you'd be able to use any JDBC or ODBC Database tool such as DbVisualizer or SquirrelSQL. Those are GUI tools; I've not come across a JDBC command line tool but there may be one out there.
Alternatively, there is an Instant Client version of SQL*Plus which will give you an Oracle sqlplus command line without a full Oracle install.
Oracle's free PL/SQL IDE SQL Developer can use Type 4 JDBC drivers to connect to a number of different types of databases, obviously including Oracle. You can use that without doing any sort of Oracle client install if you so desired.
You could try the Oracle IDE SQL Developer which is a free download. It is written in Java and is available for both windows and linux. It is self contained and doesn't require the Oracle client be installed.