In many places around the United States, localities and organizations have been printing money as a means to drive business toward local establishments and raise money for the community in times of economic hardship. Perhaps the most well established example of this are Berkshares in the Berkshires region of Massachusetts. Berkshares are slightly less valuable than U.S. dollars (you get 100 for $95) and can be obtained at a number of local banks and used in local stores. The IRS even gets in on the act and taxes Berkshares purchases, at their established "exchange rate".
However, Article I, section 10, clause 1 (The Contract Clause) of the United States Constitution says explicitly:
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Clearly the Berkshire region is not a state, but the rather explicit endorsement of the currency by the local government, and the local government's recognition by the state of Massachusetts seems to give the currency the tacit approval of the state. As such, how can the alternative local currency, working to devalue the U.S. dollar in the region, be constitutional?