ImageCache allows you to setup presets for image processing. If an ImageCache derivative doesn't exist the web server's rewrite rules will pass the request to Drupal which in turn hands it off to ImageCache to dynamically generate the file.
- Install and enable ImageAPI module
- Install one of the ImageAPI toolkits (preferably GD)
- Clean URL support or Private Files (more info)
- ImageCache Profiles - Manipulate user profile images
- iTweak Upload - Enhances the Drupal-core Upload forms and Comment Upload forms.
- Taxonomy Image - Upload images for taxonomy terms
- Ubercart - An excellent e-commerce system for Drupal
- Imagecache Actions - Provides additional, advanced actions like watermarking
- ImageMagick raw action - Lets you create custom ImageMagick actions
- Transliteration - Helps avoid problems with file names with spaces in their names
- ImageField Focus - Scale and crop images to focus on their important portions
- ImageCache External - Use ImageCache with external images
ImageCache has been moved into core in Drupal 7.
Please see the road ahead.
If you use the Lighttpd web server please read this.