Notepad++ is a text editor with extra functions for programmers and web developers. It has many features such as: a tabbed mulitple document interface, ability to create text macros, auto-completion, search and replace in multiple files and syntax highlighting and code folding/outlining (allowing you to collapse and expand sections of code/html in order to focus on the parts of your document that require attention) for nearly 50 programming languages including C/C++, Java, HTML, PHP, CSS, XML, Visual Basic and Perl.
Notepad++ also supports plugins which extend the functionality of Notepad++ in many ways.