Check my recommendation on Sword of the Stars: The Pit firstly before reading this one. So you may have been playing a lot of Sword of the Stars: The Pit and prehaps you've reached the end of the pit.... or perhaps not...
Anyway, Sword of the Stars: The Pit - Mind Games is the first expansion to the game and it doesn't dissapoint. Adds 2 new classes: Solforce Psion and the Tarka Ranger.
Solforce Psion uses the main features of the expansion pack which are psionics. Psionics are similar to magic in other RPGs and this expansion has a lot of them. None of them are new or original, but fun to use nonetheless. Problem is, due to difficulty and mechanics of the game, you may not be able to utilize many of them unless you use the Solforce Psion. However, test your luck on the pit.
The Tarka Ranger is more of an Assasin character and the only non-human character. She's very fun to use, more than the Marine because her atributes are more flexible and more upfront character.
Overall, Sword of the Stars: The Pit - Mind Games makes the game more complete, more fun and HARDER!
So if you're gonna pick up Sword of the Stars: don't forget this expansion! I recommend!