Boost C++ Libraries

Recently, I checked some job ad. on the web and found some jobs require knowledge about "Boost". I have not heard "Boost" before. After some googling, I visit the web site of Boost. Boost is really a great thing. It is a set of C++ libraries contains many useful programming constructs with good documentation, e.g. smart pointers, regular expression, object pools, state machines.... Lastly, it is open source and free.