XBM is a very standard and popular format for monochrome bitmaps. You can find XBM bitmaps (also known as X11 Bitmaps) almost everywhere you look. The bitmap(1) program is a pretty good editor for creating XBM files, although it doesn't have any nice features.
XPM is becoming the standard and popular format for multicolor icon pixmaps. You won't find very many XPM files lying around, although several archives of them exist. Xpaint(1) can load and save XPM files (add graphics to run xpaint). XPM files contain nice features like being able to specify different colors for grayscale, color, or monochrome monitors, etc. For more information on the XPM format, look at /mit/graphics/src/xpm-3.2g/doc/xpm.ps with your favorite PostScript viewer, or print it out. Converters exist for converting other kinds of graphic files to and from the XPM format via the PBM suite of programs. PBM is a simple monochrome storage format which was designed specifically for easy conversion between other formats; PPM is the multicolor version of PBM. For instance, there exist programs to convert GIF, TIFF, and many other formats to PPM (giftoppm, tifftopnm, etc.). The PBM suite is in the graphics locker. Type man pbm, man pgm, man ppm, and man pnm to find out more about this powerful suite of programs. There exist converters between the PPM format and the XPM format, xpmtoppm and ppmtoxpm; these are also in the graphics locker.
Finally, the McBitmap format was homegrown for xzewd 1.0, before I knew the XPM format existed. It is more compact than the XPM format and can also have transparent colors, different colors on different kinds of monitors, etc., but is incredibly ugly. Nothing loads or saves McBitmap except xzewd and Clux (another program I worked on), but there do exist three McBitmap utilities on DECstations. mcb is the general McBitmap tool; it lets you create McBitmaps by overlaying several XBM bitmaps, `explode' a McBitmap into XBM's, change a McBitmap's colors, etc. man mcb for more information. Also, there are conversion programs to and from the PPM format (see above): mcbtoppm and ppmtomcb. These also have man pages.