With Spore having some troubles and being not exactly the most complex game, I would like to point out very quickly a neat alternative: Q-Pop. The game is VERY old. I think back from something around 1995. It is also technically not the most amazing one. It is partly turn-based, you can’t customize the appearance of creatures, it’s obviously not online and it only covers the creature stage of Spore. On the other hand, it has more strategic depth then Spore. You can change the attributes of your creatures and they DO affect the survival rate of your species. You can also play it multiplayer at a single PC and the best part is that it is free and only 2,5 MB in size (the game is avalible only in German, sorry).
But it’s a very good game