PSPad editor has been designed to allow editing several files at once in different programming languages. It's a very simple but powerful tool that allows you to work with several documents simultaneously in different tabs, save sessions, compare texts highlighting differences with colors, spelling and the use of code templates (HTML, PHP, Pascal, JScript, VBScript, MySQL, MS-Dos, Perl , etc..).
It also provides direct access to HTML tags, the preview tab in Internet Explorer and Mozilla Firefox, etc so you can see how things are going easily. It also contains a hex editor, an ASCII table, an internal browser with Apache, code explorer for Pascal, INI, HTML, XML, PHP.
For a free editor, this is an incredibly powerful and effective solution for any serious programmer.