"A little hypertext system" - with an improved version 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.
TCL Program RamDebugger has a help system based in tkhtml where the help is included as html files in a directory tree. Supports full search.
How to Define Hooks for Context Sensitive Help
Would Notebook be a good basis for a Tk help system?
Christian Werner's winhelp [L1 ] simplifies interaction between Tk applications and the native Windows help system infrastructure.