Classic Traveller... free!

Specifically, the Starter Edition of the Classic Traveller line is available for free at both DriveThruRPG and RPGnow.  This was originally done as a Christmas promo, but apparently has been allowed to continue.  If you've never played Traveller in its original incarnation, this is an excellent way to introduce yourself to one of the true classics of the early days of RPG's.

There are three books that comprise the Starter Edition (rules, charts & forms, adventures); be sure you download all of them.  Then break out the d6's, and get ready to start travelling...