Is there such a thing in Mathematica; something like line numbers in Notepad++?
In .nb (notebook) files there is no such thing as a line number, at least not in the standard Mathematica FrontEnd. However, there really should be. Why? Because it would be so much easier for us poor programmers who unfortunately cannot program all relevant lengthy code in normal .m packages (because a customer sometimes actually does want to use the FrontEnd, for understandable reasons like documentability), and with line numbers it would be easier to document code, since with the standard (* *) type comments one cannot do it problem-free.
Go to Preferences->Evaluation and click the line that says: Show In/Out Names. It's not exactly the same as line numbers -- rather, it's cell numbering but can serve some of the same functions.