The Sims 2: Open for Business (PC Games, 2006)

Great expansion pack--it adds a ton of new stuff to the game. Opening businesses is challenging, fun, and addictive, especially because you can basically do whatever you want (sell almost anything available in Buy Mode or make things yourself, plus restaurants and more...). Aside from the businesses, the game also adds loads of new skins and interesting building options such as elevators and split-level foundations. It's very refreshing for a veteran Sims player, and for a more casual player it's just a lot of fun. This is easily my favorite expansion pack to date.