[ from http://alleg.sourceforge.net/license.html - mkennedy ]

The giftware license

Allegro is gift-ware. It was created by a number of people working in
cooperation, and is given to you freely as a gift. You may use,
modify, redistribute, and generally hack it about in any way you like,
and you do not have to give us anything in return.

However, if you like this product you are encouraged to thank us by
making a return gift to the Allegro community. This could be by
writing an add-on package, providing a useful bug report, making an
improvement to the library, or perhaps just releasing the sources of
your program so that other people can learn from them. If you
redistribute parts of this code or make a game using it, it would be
nice if you mentioned Allegro somewhere in the credits, but you are
not required to do this. We trust you not to abuse our generosity.

By Shawn Hargreaves, 18 October 1998. 
