LaTeX: how to highlight some bibliography entries by adding an asterisk before the entry?

Author-year citation style is assumed.

If a bibliography is prepared manually it is as easy as to precede each entry with say \bibAst command. Example:

\newcommand\bibAst{\leavevmode\hbox to0pt{\hss$^\ast$}}

\harvarditem[Bagozzi and Dholakia]{Bagozzi and
  Dholakia}{2006}{bagozzidholakia06}
\bibAst\textsc{Bagozzi, R.~P.,  {\small and} U.~M. Dholakia}  (2006): ``Open
  Source Software User Communities: A Study Of Participation In {L}inux {U}ser
  {G}roups,'' \emph{Management Science}, 52(7), 1099--1115.

If one insists on using BiBTeX one has to mark records which are to be highlighted. It can be done with an extra field (named highlight in the example below):

@Article{bagozzidholakia06,
  title	= "Open Source Software User Communities: A Study Of
   Participation In {L}inux {U}ser {G}roups",
  author	= "Bagozzi, Richard P. and Dholakia, Utpal M.",
  journal	= j-msci,
  pages		= "1099--1115",
  number	= "7",
  volume	= "52",
  year		= "2006",
  highlight     = "Y"
}

Fields unknown to BiBTeX's particular style are omitted, so highlight cause no harm when processed with standard BiBTeX style.

One need to modify BiBTeX style now. This is a two-step procedure. First one has to add highlight to the body of ENTRY function:

ENTRY
 {
  ...
  highlight
 }

Next one has to modify format.authors function. Exact form of a format.authors function is style-dependent. In case of apalike the code looks like:

%FUNCTION {format.authors}
%{ author empty$
%    { "" }
%    { author format.names }
%  if$
%}
% change to:
FUNCTION {format.authors}
{ author empty$
    { "" }
    { highlight empty$
       { author format.names }
       { "{\bibAst}" author format.names * }
       if$
    }
  if$
}

BTW: the meaning of some BiBTeX expressions are as follows: S1 S2 * denotes S1 S2 strings concatenation; L F1 F2 if$ denotes logical if (execute F1 if L is true otherwise execute F2).

The contents of highlight is irrelevant. Sole existence of highlight denotes that the record is highlighted.

NOTE: the style should be more elaborate if some BiBTeX entries starts with something other than authors names.