"A little hypertext system" - improved version is in htext
Steve Landers has begun using a Wikit for a hypertext help system. To learn more, start up a copy of a current (empty) wikit and look at its Adding Help to Scripted Documents page.
Keith Vetter's SpiroGraph embeds a "nicely done" (according to David Welton) "simple dialog for help".
See the iwidgets hyperhelp widget: hyperhelp
Some people have "tooltips" in mind as the canonical help model.