Currently, the Federal government offers a 30% tax credit to homes and businesses that install a solar hot water system.
Several state and local incentives also exist throughout the U.S. to encourage renewable energy. The state of Massachusetts for example, offers a $1000 personal tax credit for homeowners that install a solar hot water system. On average, the cost of installing a solar hot water system is reduced by approximately 30-50% after government rebates & incentives.
The following link will direct you to the Database of State Incentives for Renewables & Efficiency (DSIRE). DSIRE is a comprehensive source of information on state, local, utility, and federal incentives that promote renewable energy and energy efficiency: http://www.dsireusa.org/