A picture is worth a thousand words, so without further delay:
[http://www.xmission.com/~georgeps/cscrollbar/cscrollbar-2.1.jpg]

(Newer versions have 4 arrows (two for the top and the bottom))

License: BSD (Tcl style but copyrighted by GPS)

Platforms: all supported by Tcl/Tk

[GPS]: Cscrollbar has improved with age.  I use it everyday in my editor, and other applications.  Stepwise Refinement is what I followed with this (without knowing the specific term until afterwards :).  It supports the Tk scrollbar protocol, so every standard Tk widget that supports scrolling can use it.  The code is concise, and works well.  There is a rough area in the drawing of some gradient colors, but now that I've written new code for [Drawing Gradients on a Canvas] I should fix this problem soon.  It works well for many colors though as is.  If you find a feature lacking let me know and I can help, or send me[mailto:GeorgePS@XMission] a patch.