XV displays one image at a time in an output window, or on the root window. You can arbitrarily stretch or compress the window, and the picture will be rescaled to fit. You can rotate the picture in 90-degree steps. You can flip the picture vertically and horizontally. You can repeatedly 'crop' a picture (define a rectangular 'region-of-interest' and 'throw away' the rest). You can magnify any portion of the picture by any amount, up to the maximum size of your screen.
XV allows you click on the picture to determine pixel RGB values and x,y coordinates. You can perform arbitrary 'gamma correction' on the picture both in RGB space and HSV space. You can specify the maximum number of colors that XV should use, for some interesting visual effects. You can have the program produce a stippled version of the picture using black and white, or any other pair of colors.
XV can write images in a variety of formats, with many of the modifications you may have made to the picture saved as well. You can use XV to do format conversion. XV will also automatically uncompress compress-ed files, as well as read files from stdin.
The full xv manual, in postscript format, can be found in /usr/local/doc/xvdocs.ps.