Generally speaking, the credit is for 10% of the improvement cost, up to a lifetime credit of as much as $500. A tax credit is a direct reduction of tax liability, unlike a deduction.
For Energy-Star qualified windows and doors, 10% of the materials cost (not including installation) qualifies, up to $500 for doors or $200 for windows.
Insulation, metal and asphalt heat-resistant roof replacements: 10% of materials, up to $500.
Biomass stoves and qualified water heaters: $300.
For heating, air conditioning, and ventilation systems, the credit varies from $50 to $300.