"What's this?" help or context-sensitive help* used to be commonplace in Windows applications a few years ago. Dialog windows had an additional "?" button in the top right, that when clicked, gave you an arrow cursor with a question mark. When you then clicked on a control, you would get a popup with an explanation:
Alternatively, you could right click the control and select "What's this?" from the context menu. In non-dialog windows, you either had a toolbar button, or an item in the help menu to get the help cursor. And on Mac keyboards, there is actually a help key that gives you a question mark cursor. (I don't know if it ever worked in this way, however. It works in no application on my Mac.)
I find this feature immensely useful. Especially in complex configuration dialogs (browsers, office programs, system utilities), where you often have lots of checkboxes with cryptic labels. It's also helpful if you have a strange toolbar icon and want to know what it does.
Why did this kind of help become unpopular? What are arguments for and against it from a UX perspective? Should I provide it in new applications?
Personally, I think the half-hearted implementation of "What's this" in many applications was one cause of it's demise. A lot of programs would open the help window (Windows help) when you clicked the question mark or selected "What's this?". That takes a while, creates a distraction, and sometimes even moves your windows around (in case of Office). Since this is the opposite of the unobstrusive tooltip I wanted to see, that discouraged me of trying this at all (as a user).
*) Context-sensitive help is now used in a much broader sense, e.g. opening a webpage with a description of the current dialog. However, I remember Windows books specifically refering to "What's this" popups as context-sensitive help. By the way, in other languages (e.g. German), the popups are refered to as "direct help", which I find much nicer to write than "'What's this?' help".