Sum contents in column from starting cell on down without setting an explicit last cell index

For Excel 2003 or before:

=SUM(B5:INDEX(B5:B65536,MATCH(TRUE,INDEX(ISBLANK(B5:B65536),0,0),0)-1,0))

For Excel 2007 or after:

=SUM(B5:INDEX(B5:B1048576,MATCH(TRUE,INDEX(ISBLANK(B5:B1048576),0,0),0)-1,0))