I like great tools. An efficient tool will help you do a piece of work faster than when you are on your own. If the tool is made by someone who is close to the material chances are the tool is efficient. The tool I use most is SQL Developer. It is easy to use and I promote it to any developer where I work, especially if they only need a quick access to the data dictionary, run some simple (or complex) SQL statements. It is not invasive, contrary to other tools it does not provide all the DBA tools that may be useful in some cases, but also may be rather intrusive and put a severe load on oracle instance as it queries the internal v$-tables.
Installation of SQL Developer can hardly be easier; unpack a zip-file, and execute the program. It asks for the java.exe if you downloaded the version without java and asks you if you would like to migrate from a previous version. Connection can be done through JDBC, meaning no oracle client is necessary, but in case you have the client software installed, it lets you pick a connection from a list of known tns-connections.
Latest version 1.5 adds support for CVS and Subversion, and lets you generate documentation in html-format. Very useful for offline-browsing or add to some intra net server.
Another tool I use is Schemester. Simple and efficient to capture a data model from the database and visualize the foreign keys. Schemester used to be located at www.schemester.co.uk, but that site seems to have been abandoned, and I have not seen much activity around it for some time now. As you upgrade or change the data model in Schemester it can generate "delta DB DDL script". On top of my wish list for SQL Developer are these features from Schemester. If the status quo sets the standard they are likely to be implemented without some bugs that Schemester has.
Sue Harper posts updates about SQL Developer on her blog, you won't have too many posts to catch up, but the tidbits are absolutely useful (like when I can look forward to a new version.)