SciTE is a free, opensource editor. This is by far the best editor I have ever used.  It has a tabbed interface, code highlighting, explorer integration and it is updated often.

Before I go any further, I should mention that I have baked my own version of a SciTE installer.  It's built with WiX which means that it's a nice friendly MSI.  I have included Andre Burgaud and Angelo Mandato's fix to the explorer context menu for opening files in new tabs (as against new windows).  This installer makes the over-all SciTE installation experience far simpler.

Feature-for-feature you will probably find other editors which can outperform SciTE, but none can outperform SciTE's loadup performance, and to me this is by far the most important feature of any extended editor.  I have tried to use other editors only to be painfully waiting for the software to loadup when all I want to do is do a really quick change or search of a file.  Multiple quick changes/searches become a nightmare of waiting when using other software - not with SciTE.

The code highlighting is very handy when you want to review a piece of code without having to loadup a full-blown IDE.  All code highlighting is customisable (normally automatic for most code files) and more known extensions are being added to the list all the time.  There are indentation guides, an auto indentation fix tool and even compile/debug/run tools.