Mars

Ever since I read the War of the Worlds and Princess of Mars I have wanted to have a D&D game based on Mars/Barsoom.  This was heightened when I later read the Martian Chronicles.

It turns out that the real Mars is far more interesting.

The latest Martian rover Curiosity has landed successfully on the Red Planet. The stated goal of this little robot is to see if Mars was ever capable of supporting life or even had life at one point.

I should not have spell out the ramifications of a positive discovery would have in both science and religion.

Till that happens you can follow Curiosity's Twitter feed. https://twitter.com/MarsCuriosity
And it's official site here: http://mars.jpl.nasa.gov/msl/

While Curiosity roams the face of the real Mars, you can roam the face of the Mars that never-was with these games.

Mars
Savage Worlds Edition
d20 Edition

Adamant Entertainment distilled some of the best features of the Planetary Romance/Sci-Fantasy genre into their Mars books.  The lineage is obviously Edgar Rice Burroughs, with Green, Red and White (Ape) Martians.  There is also a fair enough amount of H.G. Wells, but I have a hard time seeing this dying Mars invading Earth.  As they advertise this is not the Mars of reality, this is the Mars that never was.  This is Barsoom as it were.  While not "John Carter of Mars the RPG" it can be played that way.   There are even some surprises in the form of the Grey Men of Mars.  Hint, they are not the "Greys" of later UFO mythology.
There are plenty of options for characters with an emphasis on high heroism and great feats.   Imagine all the adventure of Victorian Times and the Pulp Era with the feel of a Space Opera in a D&D campaign then you get an idea of what Mars can do or be.  This all reminds me a bit of the "Dying Earth" genre as well, since Mars is dying.  Maybe that invasion of Earth is not too improbable after all.
NOTICE: Adamant Earlier today decided to offer these at half off!

Space 1899: Red Sands
This is the Savage Worlds update to the classic Space 1899 series.  Like it's fore-bearer this is a game where brave men and women from Earth brave the Ether to travel to a dying Mars or a Venus covered in lush jungles and dinosaurs.  Based on the works of Burroughs and Verne this is a space travel game with a twist.  There is plenty of room for adventure and the importunity to plant the flag of the British Empire on a new world or even find adventure of your own.
It is the Savage World rules and you need the core rules to play this. It is great fun and it is to date the best reason given to me to play Savage Worlds.
The only downside to this is that there is no conversion notes from the old Space 1899 to the new system.