Each source file contains the following notice:

//
// Copyright (C) 2004-2006, Maciej Sobczak
//
// Permission to copy, use, modify, sell and distribute this software
// is granted provided this copyright notice appears in all copies.
// This software is provided "as is" without express or implied
// warranty, and with no claim as to its suitability for any purpose.
//

The above is the shortest license I have found.
I have copied this wording from one of the source files of the Boost library.

Just to explain the above terms:

1. The library is free (like in "freedom").

2. You can use it for any purpose you wish. You can even sell it.

3. You can modify the source files. If you do this, please feel free to add
   your own copyright notices wherever you want, if you want.
   You may also add more restrictive license terms.
   You only have to keep the above notice unchanged in all source files.

4. You can combine this library or its modified version with other software.
   The license claims nothing that would affect the resulting work.
   See point 3. if you need to make any modifications.

5. You can compile this library or its modified version to the binary form.
   The license does not affect the compiled result in any way.
   In particular, you do not need to (but you still can) display the above
   copyright notice to the final user, and you do not need to (but you still
   can) inform the final user that your product was prepared with the help
   of this library.

6. Do not sue me for my (and your) mistakes and errors.

As you see, this license is very permissive.
