ACT files are Adobe Color Table files, which are color tables for the image editing software Adobe Photoshop and the vector graphics software Fireworks. Act files contain so-called indexed colors and serve as CLUT (Color Lookup Table).
For example, color table files are used in webpage image preparation to store images in web-safe colors. For example, if you choose 'save for internet', you can load an ACT color table to colorize the image.
ACT files can be created just like aco swatches create files. You can access the color tables in Photoshop by clicking on the little arrow in the Swatches window. Photoshop also offers a few included color tables, including Windows, Mac or Black and White.
Note, however, that indexed colors are only supported by gif and png-8, but not by jpeg or bmp.