Norton Guides. A similar TSR that also included a compiler for building your own guides. I remember one popular one was the Ralph Brown Interrupt List. I think about this tool a lot, but I think IDE's with auto completion and showing parameters has largely replaced the need. But using NG, still seems faster than having to launch an IDE help system or doing a google search.
Circa 1990, a DOS TSR (remember those?) that when you pressed Alt+H (I think) popped up help for the API call at the cursor (usually OS/2 or Win16).