Syntax Highlighter with linenumbers. Also has the ability to load syntax and colors at runtime, which makes it great for multi-language code books.
## EDIT ##
I managed to get the tabs to work nicely using Paul Caton's Keyboard Hook (https://www.vbcoders.com/vb/scripts/ShowCode.asp?txtCodeId=42918&lngWId=1)
For the next version I am looking to have Undo/Redo capabilities.