Like old school D&D is my favorite fantasy RPG, old school Traveller is my favorite science fiction RPG -- Classic Traveller to be specific. Mongoose Traveller is an updated version of Classic Traveller. Surprisingly, I find Moongoose Traveller to be an acceptable substitute for Classic Traveller. All the rules one needs to play in one 190 page book.
While there are lots of minor and middling differences between Classic Traveller from the 1970s and Mongoose Traveller, there is only one (immediately noticeable) major difference: In Mongoose Traveller, your character isn't likely to die during character generation. Instead, various bad things can happen to your character. Some people hate this, however, I suspect more people hated having characters die during character generation -- especially as frequently as they did in Classic Traveller.
I'm writing this post because the Main Rulebook -- which is the only book you really need to play or GM a game of Mongoose Traveller -- is on sale in PDF format on RPGNow at 75% off through June 2nd. This means the regularly $19+ PDF is on sale for only $4.80. Less than $5 for Traveller is a great price. If you've ever wanted to try Traveller, this is a great time to buy the Mongoose Traveller Main Rulebook.
If Traveller isn't your game, the following popular RPG rulebooks are also on sale for 75%:
* Legend of the Five Rings 4th Edition (Sale Price: $8.75)
* Shadowrun: 4th Ed. 20th Anniversary Core Rulebook (Sale Price: $3.75)
* King Arthur Pendragon: Edition 5.1 (Sale Price: $5.00)