| Carcassonne: Hunter's and Gatherers. The five card "roles" expansion. Gives each player a special power or special tile. Its a simple expansion, but its a nice way to open up the game a bit. And unlike regular Carcassonne, its not the first step down a path of never ending expansion purchases.
Zooloretto. The Aquaretto expansion/new game. Its kind of both. Its a stand alone game that's compatible with the original. If you like Zooloretto, and want to try it as a heavier game instead of a family game, you'll like the combination game.
Transamerica. The expansion that gives you colored train tracks that only you can use. This allows more tactical play. Now you can estimate where you think your opponents are hoping that you'll go, and try to make it harder on them to use your lines. Its a whole extra level of analysis.
Cash & Guns: Any expansions. Basically they add more cash, different guns (and knives, etc, etc), and more superpowers. If you use Cash & Guns for pickup games with nongamers who are unlikely to play it more than once every year or so, then you don't need expansions. If you play with gamers, get expansions. | 
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