I thought I'd mention that Traveller (Starter Edition) is free from DriveThru RPG.
It was first published in 1983, as a reprint of the Deluxe Edition of Traveller which was no longer in print. Both editions came in boxes. This was the boxed era of RPGs after all. Traveller was also published in hardcover at that time as a standalone book, which seemed very odd to do at the time. What?! No box?!
NOTE: These rules are considered very classic. They are very abstract compared to those found in the more fleshed out Mongoose Traveller Core Rulebook.