There are a great many mathematical sites available containing everything from entertainment to the latest research. The following lightly commented list is for exploration.
Most pages have additional links, of course.
Graphics, graphical proofs, etc. Knot a Braid is maintained by Camel, the Canadian Mathematical Society's electronic service. It has links to many great math sites of various kinds. The Mandelbrot Set and Julia Sets from SUNY Binghampton. JAVA Gallery of Interactive On-Line Geometry is also from the Geometry Centre.