Today I played a game of Carcassonne during the lunch break at work. We used the free print and play expansion called "Well and Donkey". You can download this expansion from the Carcassonne page at BoardGameGeek.
Well and Donkey is a small set of extra tiles. Theres an interesting mix of fairly standard tiles. There are also 2 special city tiles that feature a Well and a Donkey on each. These special tiles dont score you points but rather allow you to poison cities. When a city contains a Well and Donkey tile it scores no points at all.
This has a tremendous effect on the way you build cities. We stack these cards in their own pile, it's not worth shuffeling them into the decks as they are obviously home made. However in the deck, there are only two city killers. So if you take one, you dont know if you'll be able to use it to kill your opponents city.
In the first few turns players are reluctant to start cities because the W&D tiles are still out there but at the same time you want to build cities for your farmers. It's a nice balancing act. And if two players get a city of more than one tile then players start grabbing the W&D tiles to poison their opponents or to protect their own cities.
It's a nice and simple expansion and if you like the base game I reccommend you try making up this expansion.