MACRS, or Modified Accelerated Cost Recovery System, is currently the method the IRS has approved for companies wanting to accelerate depreciation on business equipment. MACRS includes an asset classification system which shows the number of years of depreciation for each type of asset.
IRS Publication 946 "How to Depreciate Property" (see Chapter 4) has a general asset classification system and a MACRS Worksheet.
Wikipedia "MACRS" article has a more complete asset classification list.
