The versions 0.5, 0.6 and 0.7 were a very early editions of Strawberry Prolog and we did not recommend them for serious work. They are free for use and distribution (including business purposes), like 1.0.
This version 1.0 is the first version which is good enough even for professionals. This will be the last absolutely free version. In our opinion it is already the best Prolog compiler in the world!
Versions after 1.0 will be in three editions, Light, Middle and Professional.
The Light edition will be freeware. The professional and middle editions
will be available for the ones who made subscription for Strawberry Prolog.
This subscription includes the current version and all future versions
including 4.0 . When you bought a subscription you will receive
a personal password which can be used to download all future versions including
4.0 . The cost for this subscription for the moment is only $249 USD
for the Professional edition and $98 USD for the Middle edition
(look at buy a subscription ).