Other things that can and have been traded in Settlers of Catan when I play with my friends:

Future production. Your next resource, next specific resource, all resources for next X rolls of the die, resources from a specific tile, or settlement/city.

Future rights. Right to trade at a certain rate in the future. Right to choose robber placement in future.

"Equity" in a settlement. Contribute resources to another player building a settlement in exchange for some fraction of the resources it will produce in the future.

And I'm sure as we keep playing more things will be added to this list.

For a slightly silly but still fun variant. Settlers with Nuclear Weapons! http://albertsun.info/misc/nuclear-settlers.pdf