Classic Traveller is an amazing scifi role playing game dating back from 1977 and still played today - a 5th version is out. Since today is FreeRPGDay, I guess Marc Miller decided to make this one free.

“This is the original text of the 1981 edition of Classic Traveller based on page image Scans. It includes Books 1-2-3, with errata and corrections inserted (where possible; and additional material in an errata appendix). The PDF has been OCR’ed. Margins are upgraded to 6x9 (from 5-1/2 x 8-1/2) for better margins.”