University of South Florida Digital Mammography Home Page - Provides free access to a huge database of high resolution lossless JPEG images of mammograms. The database contains approximately 2,500 studies. Each study includes two images of each breast, along with some associated patient information (age at time of study, ACR breast density rating, subtletly rating for abnormalities, ACR keyword description of abnormalities) and image information (scanner, spatial resolution, ...). Images containing suspicious areas have associated pixel-level "ground truth" information about the locations and types of suspicious regions. Also provided are software both for accessing the mammogram and truth images and for calculating performance figures for automated image analysis algorithms.
The Mammographic Image Analysis Society MiniMammography Database - A free, lower resolution subset of the full MIAS database of mammograms. Images have been reduced to a 1024 x 1024 resolution, and are in simple PGM format. A free viewer / converter for PGM images is XnView, who is linked next.
XnView - A free, well-ported viewer and converter for something like 300 different image file formats. I use this and Adobe Photoshop for all my image work.