Clouds

• The Cloud Appreciation Society
  "At The Cloud Appreciation Society we love clouds, we're not ashamed
  to say it and we've had enough of people moaning about them."
• Extreme Instability
  Some of the most truly awesome cloud photography by storm chaser Mike Hollingshead.
• The "Karlsruher Wolkenatlas"
  
  

Fractals

• Apohysis
  Mark Townsend's wonderful fractal flame editor. Totally addictive and free.

Geometry

• Paul Bourke
  A diverse encyclopedia of practical information for those who appreciate
  the beauty of geometric curves and parametric shapes

Raytracing

• Oyonale
  Gilles Tran's " Expériences graphiques"
• POV-Ray
  Freeware raytracer with a longtime and devoted group of followers.

Software - some favorite little programs

• Axialis IconWorkshop
  a beautifully designed icon editor for both Mac and Windows new Vista icons
• AutoHotkey
  a free, open-source Windows utility for automating keyboard and mouse routines
  
• AVG
  good, well-behaved antivirus freeware that doesn't try to bug your system with itself
• 7-Zip
  easy to use zip utility (free)
• Color Schemer ColorPix
  a neat little app that will let you grab the color values of any pixel on your screen (free)