Notepad++ - XML Source Code Color Editor
Notepad++ is a free program download and a source code editor which you can use to edit the template XML file of your blog. It features a multi-color highlighting scheme so you can easily know the start and end of html tags used in your template. Multiple tabbed windows are also an additional feature.
In Settings menu, Style Configurator, under Select theme:, choose Vibrant Ink which is my favorite color template. Notepad++ also has a line code guide although when you encounter an error in Blogger's HTML editor, it doesn't coincide with the line number.