I recently stumbled over this little gem of a program called Programmer's Notepad. It's likeweight in the same sense as Notepad, but it has a bunch of features that are useful for programmers.
These features include syntax highlighting, regexp search and replace, code folding and a lot more. I've been using EditPad Pro for a while, but it's a bit heavy on resources (it has more features though) compared to Programmer's Notepad, so I decided to ditch it. I now use it all the time for writing both XHTML, CSS, PHP, Perl and Tcl. Oh, and did I mention it's free?