Oracle has a pretty good web based tool for executing queries against the database called iSqlPlus. It is quite similar to Sql*Plus, but is better suited for testers than DBAs. In Sql*Plus, users complained about long lines that would get wrapped, thus messing up the output. Since iSqlPlus works within your browser, it builds an HTML table containing query results with a width just enough to display all the results. It also keeps a history of previously executed statements for the sesson, so you can easily execute a previously executed query.
For a default install of Oracle, you'll generally find iSqlPlus installed at http://localhost:5560/isqlplus . Do add your comments to this post if you've had a chance to use both.