It already has a plugin for Spell Checker which you can find under Plugins->Spell Checker ..
All it requires is GNU Aspell and a dictionary installed on your machine.
GNU Aspell is a Free and Open Source spell checker. It can either be used as a library or as an independent spell checker. Main features include
- possible suggestions for misspelled words
- supports UTF-8 document encoding
- supports multiple dictionaries.
Enable Spell checker in four easy steps -
1. Download and install Aspell for Windows from http://aspell.net/win32
2. Now install a dictionary which you can download from the same website.
3. Notepad++ requires a relative path for aspell-15.dll. Open Plugins->Spell Checker.. A dialog box will open where you can edit the relative path. I have both Notepad++ and Aspell installed in "C:\Programs Files" so I entered the path as "../Aspell/bin"
4. Restart Notepad++
Bingo! Now if you open Spell Checker, it will show you the list of misspelled words in the current document and possible suggestions.