What are common contracts in Catan?
We don't use contracts with the group of friends I play Catan with.  While it's an interesting idea, trading resources for nothing in return during the trade is explicitly against the rules of the game.  Also, the player who's turn it is has to be involved in all trades being made during that turn.  So, a player giving resources to a player they promised them to previously would break two rules in the game.

I could have some of same problems as allows players to bribe off the robber if players start trying to extort resources from each other..  It would be easy to abuse, and adds complexity in the game since you would have to track contracts.  Players would have to be to surrender the resources promised when they generated them in order for this to work.  I personally don't think the added complexity would make the game better.  It would certainly deepen the trading aspect of the game though.

For those not familiar with the topic of bribing off the robber, we discussed this previously in, Is bribing off the robber a good addition to Settlers of Catan?