Shimano Hollowtech II Tool (TL-FC16 replacement)

This tool can be used to loosen that Torx-like cap, which is holding the left crankarm on the bottom bracket spindle (see picture).

This is needed to swap/clean your crankset or replace the chainrings.
The video here explains how to remove your crankset
http://www.youtube.com/watch?v=hs_ywOIYzdY
There are plenty of instructions about this procedure online.

Don't be worried about this tool being made out of plastic - the original is as well.