You can use The Oracle Enterprise Manager to monitor the active sessions, with the query that is being executed, its execution plan, locks, some statistics and even a progress bar for the longer tasks.
See: http://download.oracle.com/docs/cd/B10501_01/em.920/a96674/db_admin.htm#1013955
Go to Instance -> sessions and watch the SQL Tab of each session.
There are other ways. Enterprise manager just puts with pretty colors what is already available in specials views like those documented here:
http://www.oracle.com/pls/db92/db92.catalog_views?remark=homepage
And, of course you can also use Explain PLAN FOR, TRACE tool and tons of other ways of instrumentalization. There are some reports in the enterprise manager for the top most expensive SQL Queries. You can also search recent queries kept on the cache.
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