Galactic Civilizations II: Dark Avatar gets an A+ from Sci-Fi Weekly. It appears to be the best 4x space game around now.
Star Wars PocketModel Trading Card Game, which I am assembling to play with my nephew, only gets a C+, but the review looks very rushed. It did not even mention that Booster Packs are just that - boosters and not the whole game. You should get the $15 box to start and add boosters. (I didn't because the boxes were sold out and I even had to wait a few days for the boosters to come back in.) A custom rules game with custom map already.
Space Empires IV is the classic space game of 4X (eXplore, eXpand, eXploit and eXterminate) and has a demo and a new fantastic lower price as the updated 3D Space Empires V is out also with a demo and a great price.
Sid Meiers Civ 4 has a new expansion that is supposed to be very good if you can handle the requirements. My speed is borderline and my graphics card is subpar for it. Civ Fanactics overview. This is supposedly the largest expansion for a Civ game ever.
See also this list of Free Computer Version of Board and Card Games with Computer AI (Artificial Intelligence) Opponents. I need to check out the Excel Puerto Rico version now that I have a monitor that can handle it. Also check out abandoned by manufacturer and available as downloads computer games like Advanced Civilization at Abandonia. More abandonware links here.
While we're doing lists - Moon's best games of 2006. Twilight Struggle tops the list and I will get that soon (even if it has been decided that one side has an advantage that needs a minor rule change like bidding for sides or adjusting victory points.) I will not pay the $57 plus tax my local game store wants for it, however. I will check out a seller at BoardGameGeek or a discount online store.