1. Turn Notepad++ into PHP IDEWith the help of DBGP the php debugger (XDebug) which talks DBGP protocol transform your Notepad++ to a php IDE.
2. Auto IndentWith NppAutoIndent you can Auto indents the next line for you. Saves you some button presses when you have lots of nested conditionals.
3. Auto complete bracketsWith XBrackets Lite when you type in an opening bracket, it automatically puts in the closing bracket for you! Very handy.
4. Auto Complete CodeTo enable this feature you have to download CCompletion plugin. It is much like IntelliSense. The only problem with it is it displays the identifier in primary view, not secondary.
5. Synchronization with FTP ServerNppFTP plugin let you modify and upload files directly to a ftp server.
6. Highlight MarkerWith Change Markers plugin a colored marker will be displayed on the left side of a line you’ve just edited. It indicated which lines have changed and whether the changes have been saved. Functions exactly like in Visual Studio.
7. Switch files faster by pressing a key and typing nameThere is a plugin available in Notepad++ File Switcher - which allows you to change files by hitting the shortcut and then typing its name. Useful for when you have several files open at once and Ctrl+Tab takes too long to sift through.
8. Spell-CheckerAs a name suggest Spell-Checker plugin helps you when you’re editing normal text files.
9. Auto Close XML TagsXML Tools plugin auto-close tags and provides other useful features.
10. Compare two text files side by sideWith Compare Plugin you can find differences in two text files very easily. The differences are showed in both files by highlighting them with colours.
Bonus :Bulk Edit Text Files in Notepad++ (Without Opening Them)You don't need any plugin for this. Simply hit CTRL+SHIFT+F and browse the path where all your files are resided. You can also choose type of files as *.txt for text file or *.php for php files.
There are many other plugins like Hex Editor (Adds a Hex Editor in Notepad++) and Color Picker(translates your selected colour in hexadecimal) which can increase your productivity.
Note: To download plugins you can use Plugin Manager(under Plugin menu) in which most of the plugins will be available there or you can find all plugins on Notepad++ Wiki page.