Photoshop brush

Photoshop brush (ABR) is a file format used by Adobe Photoshop for brushes (graphics, and information about brush behavior).

Specifications
Note that the information in these documents might not apply to all versions of the format. The current version does not appear to be well-documented.
 * Photoshop File Formats Specification V6.0 Release 2
 * Photoshop Brushes (ABR) Specification

Software

 * Adobe Photoshop
 * XnView

Sample files

 * There are many websites offering Photoshop brushes for download. Some examples:
 * http://fbrushes.com/
 * http://www.libertiny.com/brushportfolio/