Here are the features of Notepad++ :
Syntax Highlighting and Syntax Folding

Supported languages :
C C++ Java C# XML HTML
PHP CSS makefile ASCII art (.nfo) doxygen ini file
batch file Javascript ASP VB/VBS SQL Objective-C
RC resource file Pascal Perl Python Lua TeX
TCL Assembler Ruby Lisp Scheme Properties
Diff Smalltalk Postscript VHDL Ada Caml
AutoIt KiXtart Matlab Verilog Haskell InnoSetup
CMake YAML COBOL D R PowerShell
Gui4Cli
WYSIWYG

If you have a colour printer, print your see more source code (or whatever you want) in colour.
User Defined Syntax Highlighting

It allows user to define his own language : not only the syntax highlighting keywords, but also the syntax folding keywords, comment keywords and the operators. ( screenshot1, screenshot2, screenshot3, screenshot4 ).
Auto-completion

For most supported languages, user can make his/her own API list (or download the api files from dowload section). Once the api file is ready, type Ctrl+Space to launch this action (see screenshot). For more information about Auto-completion, please see Auto-completion HOWTO.
Multi-Document

You can edit several documents at the same time.
Multi-View

http://notepad-plus.sourceforge.net/uk/site.htm