Notepad++ is one of the best free Text-Editors.
Here you can download a file for the Syntax-Highlightning of PLSQL here local (rename userDefineLang_xml.txt to userDefineLang.xml) or on the original site:
http://notepad-plus.sourceforge.net/commun/userDefinedLang/userDefineLang_plsql.xml
An Overview of all available Language-Files is here:
http://docs.notepad-plus-plus.org/index.php?title=User_Defined_Language_Files
This file must be located (in Windows 7) in your Profile in the Notepad++-Dir besides the normal Language-File langs.xml and must be named userDefineLang.xml.