Reda and Andreas 2008 from the National Renewable Energy Laboratory published a sun position algorithm with a reported uncertainty of +/- 0.0003 degrees.  The report is available from NREL at this link.  NREL has developed an implementation of the algorithm in C.  License information is available at this link.

Sandia has implemented the algorithm as part of its PV_LIB Toolbox function: pvl_spa.