PL/SQL is Oracle's proprietary extension to SQL. This Page identifies some of the options available if you are developing PL/SQL-based applications
Here's a short description :-
1) Use vi or JEdit and sqlplus for all your PL/SQL development, and it should work out just fine.
2) For something graphical, check out Toad or its open-source clone, Tora.
3) SQL Navigator which offers productivity gains of at least 20-50 times more than manual PL/SQL editor.
4) JDeveloper is (perhaps surprisingly) very good for PL/SQL development.
5) An interesting option is Oracle's new HTMLDB, whch is built on PL/SQL, and generates the html pages directly
from MetaData stored in the database.