Adventures like Caravan to Ein Arris are available as free downloads specifically for the 3rd Edition ruleset. Digital Format Utility Steve Jackson Games: New And Upcoming Releases
A 32-page "distillation" of the core rules, ideal for introducing new players.
The GURPS Basic Set, Third Edition, Revised is available as a fully indexed PDF for approximately $20.00.
While (often referred to as "GURPS Classic" by its publisher) has been superseded by the 4th Edition, a significant portion of its library is officially available in digital eBook formats. Current Availability & Digital Distribution