The Units4Excel spreadsheet provides:

• User Defined Functions (UDFs) to convert between any listed units, including compound SI units.
• A UDF to evaluate any function entered as text, including evaluation of input and output units.
• Recognition of all standard SI prefixes.
• An extensive list of non-SI units, based on the Wikipedia unit conversion page.
• The ability to add any other units to the list.

The download file is: http://www.interactiveds.com.au/software/Units4Excel.zip

The most recent posts with further details are:

Units for Excel 4: Scripting dictionaries
Units for Excel 5:

Function index

ConvertA Function, with alternative input options

EvalU Function