With biblatex, you could use the package option url=false to disable URLs generally and then set the internal bbx:url toggle to true for entries belonging to a certain bibliography category. Note that for the entry type @online the URL will be displayed even if you set url=false.
\documentclass{article}
\usepackage[style=alphabetic,url=false]{biblatex}
\DeclareBibliographyCategory{displayurl}
\addtocategory{displayurl}{A01}
\AtEveryBibitem{%
\ifcategory{displayurl}{\toggletrue{bbx:url}}{}%
}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@misc{A01,
author = {Author, A.},
year = {2001},
title = {Alpha},
url = {http://www.alpha},
}
@misc{B02,
author = {Buthor, B.},
year = {2002},
title = {Bravo},
url = {http://www.beta},
}
@online{C03,
author = {Cuthor, C.},
year = {2003},
title = {Charlie},
url = {http://www.charlie},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\nocite{*}
\begin{document}
\printbibliography
\end{document}
