The simplest use of hypertext as a teaching tool seems to be a question/answer
type setup.  Several web sites have lists of math puzzles, where the question
is presented on one page and the answer on another, connected by a hyper-link.
Some have hints available on yet another page.  This are a nice way for a
students to work at their own pace.  Two exciting sites with math puzzles are
The Grey Labyrinth (http://www.wx3.com/labyrinth/index.htm) and
Mind Breakers (http://huizen.dds.nl/~mahulsma/mb.htm).

A step up from simple question/answer type sites, are sites that take advantage
of the multimedia aspect of the Web.  Along with pictures and text, Web pages
can include sound, animations and interactive applications.  Mathmania
(http://csr.uvic.ca/~mmania/) makes excellent use of animations by
showing knots untying.  The Topological Zoo is a nice multimedia
reference (http://www.geom.umn.edu/locate/zoo/).  Linking Geometer's
Sketchpad drawings into a Web document is another excellent use of multimedia.

Sites which allow interaction with the Web page are another
step up.  The Geometry Center has several interactive
Web applications (http://www.geom.umn.edu/apps/) and
Java applications (http://www.geom.umn.edu/java/).
Jeff Weeks also has a few excellent mathematical Java applications
(http://www.northnet.org/weeks/TorusGames/TorusGames.html).

None of these Web pages would be as great as they are, however, if it weren't
for the fact that they're easily available to so many people.  This seems to
be the greatest advantage to using the Web to teach mathematics.

Not all attempts at mathematical Web sites are successful, though.
Because hypertext allows linking from any place in one document
to any place in another, some sites seem to be put together in a
haphazard way with no obvious organization.  Features of the Web
and hyperlinks are easily abused by the inexperienced.

On the other hand, some sites make no use of hypertext whatsoever.
The only advantage to having the lessons on the Web is the great number
of people who have access to it.

Other sites have too many large images, which increase download time.
These are not very friendly to home users who depend on slow modems
to transfer data.

Another common mistake is not allowing more than one way to view
the information.  Not all browsers have the same capabilities.
One site that DOES allow multiple ways of viewing is
The Geometry Problem of the Week (http://forum.swarthmore.edu/geopow/),
which provides a ASCII version of figure in the problem.