Myself, I feel no need to spend tons of money on marketing. So here are some free tips:
Forums - go to forums that relate to bookkeepers, accountants and tax preparers..put your site in yor sig. Now don't spam the forums, just act normal.
Articles - you can write (or have someone write) articles on the same topic as your site. Post them online.
Cards - get some those free business card that we all get email about..They really do work. Dorp them off at their offices.
that's just a few tips, There is a whole section on these forums dedicated to marketing...check it out...
if you have any question jus let me know.