These are simple to complex.  Sometimes as simple as a web site with links that you wish the students to use.

Some times they are websites with Java that do things you wish the students to see.  Like this Mandlebrot set Or this Julian set (hey it is a website so there usually needs to be some silly computer tricks)

A group of school systems along with NCA&T have joined forces to develop a series of on-line activities.  The school systems are: Lee County, Pender County, Chatham County, Rockingham County, and Thomasville City Schools.

These activities follow the format demonstrated below:  here