checking sessions to an Oracle database

Checking the maximum number of sessions:
SQL> select name, value from v$parameter where name='sessions';

Checking the number of currently active sessions:
SQL> select count(*) from v$session;

Looking for blocking sessions:

SQL> select blocking_session from v$session where blocking_session is not NULL;

Looking for idle sessions (more than 24 hours in this example):

SQL> select username from v$session where username is not NULL and status='inactive' and last_call_et/60/60>=24;

Please note that you'll need to be connected as sysdba!

Useful links:
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:209412348074#tom26247484939076