Bottom Bracket Spacer
Bottom bracket spacers are used to adjust the spindle position to add extra clearance, or to fine tune chainline. For example, if your inner chainring is too close to the chainstay you can add spacers to move the crank outward just a bit. They are really great if you want a perfect chainline on a fixed gear bike. BB spacers also allow symmetrical BBs (like the Grand Cru) to replace old non-symmetrical types (which can be hard to find) on older cranks.
Note: these will not work on Italian BBs as those have a larger diameter than English or French BBs.