Notepad++ is an open-source editor which has support for multiple programming languages, syntax highlighting, and code folding. Its handling of tabs is sane (unlike some editors I could mention). It can be configured for new programming languages.
Replace (or add to) the contents of userDefineLang.xml with this text to add coloring and folding for the D programming language. The file should be in the root directory of Notepad++. If it doesn't exist, create it. This patch supports most features of the D language, but not all. If you improve this, please modify this page so that everyone can benefit!
If putting the userDefineLang.xml in the installation root directory does not work, put it in your %HOME%/Application Data/Notepad++ instead.
If you want to be able to press a function key to compile and run your program, you can set that up via Run->Run. I made a batch file for the purpose.