Do you mean interstate contracts?
An example would be the Colorado River. The water from the river is shared by many states including Colorado, Arizona and California. Historically, California has taken most of the water, because its population needed it decades ago. Recently, however, the population of Arizona and Colorado has boomed and they are asking for more water from the river. Water rights and contracts determine how much each state and city can take from the river. The supply of water has not increased, but the number of people and farmers that rely upon it has. Therefore everyone gets less and has to be rationed so that everyone gets some.
I would say that a timely problem upon which states could come to an agreement, especially coastal states, would be the role of corporate responsibility in off shore drilling. Given the recent disaster in the Gulf of Mexico, there is much discussion about the liability and responsibility of BP and the other corporations that had a hand in building the rig and reaping its benefits. Now that disaster has struck in an amazingly brutal manner, I think that states could come to an agreement or compact as to how companies need to provide safeguards and exact levels of responsibility should something like this happen again. Naturally, the debate about off shore drilling has intensified because of this, but I think that it might be worthwhile to be able to explore what agreements or understandings that states can have with one another and business organizations in regards to off shore drilling.