Also see JPG vs. GIF vs. PNG in the Did You Know . . . ? section of Webopedia.

JPEG and GIF compression basics 
Describes compression basics for GIF and JPEG Web images, along with related links and examples.

The GIF Controversy: A Software Developer's Perspective 
Article on the GIF controversy seen from a software developer's perspective. Includes a collection of information from the history of the controversy to the most recent events.

The Graphics File Formats Page
Deep technical specifications of the most popular graphic formats.

Yahoo!'s GIF page
Yahoo!'s directory of GIF.

Yahoo!'s transparent GIF images
Yahoo!'s directory of transparent GIF images.

GIF info
A brief overview of the GIF format. The pros and cons of using GIF images within your web site. Also overviewed are the different things you can do with GIF images including creating transparent sections, animations and interlacing.