I've just found Programmer's Notepad 2, which is written pretty much the way that I was writing my own IDE. He even has middle-click to close tabs! And the find dialog is a strip along the bottom of the editing window (in the CVS version)! Just like I had done already in my IDE because those features rock so much in Firefox.

Yes, it's written in C++ with the WTL, but I'll forgive the author for that because he also writes Delphi stuff, like a Scintilla wrapper for Delphi. It is written around the Scintilla edit control that Legolas mentioned and has support for projects and project groups.