How to use multiple URLs for one bibtex reference?

Most bibliography styles format the field url direct with the command \url.

You can use the field note to add unformatted text:

 NOTE =     "\url{http://citeseer.ist.psu.edu/562256.html} and 
             \url{http://gfs.sf.net/gerris.pdf}",

you can modify the \url command with a redefinition to take two urls into account:

\RequirePackage{filecontents}
\begin{filecontents}{test3.bib}
@Misc{oai:CiteSeerPSU:562256,
  title =   "Gerris: {A} Tree-Based Adaptive Solver For The
         Incompressible Euler Equations In Complex Geometries",
  author =  "St Ephane Popinet",
  year =    "2002",
  month =   sep # "~08",
  abstract =    "An adaptive mesh projection method for the
         time-dependent incompressible Euler equations is
         presented. The domain is spatially discretised using
         quad/octrees and a multilevel Poisson solver is used to
         obtain the pressure. Complex solid boundaries are
         represented using a volume-of-fluid approach.
         Second-order convergence in space and time is
         demonstrated on regular, statically and dynamically
         refined grids. The quad/octree discretisation proves to
         be very flexible and allows accurate and efficient
         tracking of flow features. The source code of the
         method implementation is freely available.",
  citeseer-references = "oai:CiteSeerPSU:500259; oai:CiteSeerPSU:27423;
         oai:CiteSeerPSU:16254; oai:CiteSeerPSU:395363;
         oai:CiteSeerPSU:436925; oai:CiteSeerPSU:90307;
         oai:CiteSeerPSU:391348; oai:CiteSeerPSU:137987;
         oai:CiteSeerPSU:442611; oai:CiteSeerPSU:44620;
         oai:CiteSeerPSU:150038; oai:CiteSeerPSU:433875;
         oai:CiteSeerPSU:441861; oai:CiteSeerPSU:526586",
  bibsource =   "OAI-PMH server at cs1.ist.psu.edu",
  language =    "en",
  oai =     "oai:CiteSeerPSU:562256",
  rights =  "unrestricted",
  URL =     "http://citeseer.ist.psu.edu/562256.html;
         http://gfs.sf.net/gerris.pdf",
}
\end{filecontents}
\documentclass{article}
\usepackage[english]{babel}
\usepackage{url}
\usepackage[backend=biber]{biblatex}
\usepackage{hyperref}

\addbibresource{test3.bib}
\let\URL\url
\makeatletter
\def\url#1{\@URL#1;;\@nil}
\def\@URL#1;#2;#3\@nil{%
  \URL{#1}\ifx\relax#2\relax\else; \URL{#2}\fi}
\makeatother
%url = {http://citeseer.ist.psu.edu/562256.html; http://gfs.sf.net/gerris.pdf}

\begin{document}

foo\cite{oai:CiteSeerPSU:562256}

\printbibliography

\end{document}

Won't let me comment on Herbert's answer. So based on his answer:

• When you copy this into your own code, make sure you place it after you include packages like url or hyperref.

• To use spaces instead of semicolons, just replace the semicolons with spaces. This is useful for Mendeley-generated BibTeX files. When two spaces are immediately next to each other, you have to escape one so they don't disappear, i.e. ;; becomes \ (a backslash with two spaces around it). Here's a version where I additionally removed the third argument (second space) because it apparently wasn't necessary (?), and so there's no backslash anymore, either.

  \let\URL\url
  \makeatletter
  \def\url#1{\@URL#1 \@nil}
  \def\@URL#1 #2\@nil{\URL{#1}\ifx\relax#2\relax \else; \URL{#2}\fi}
  \makeatother
  

  Note that I left the "output semicolon" in there because I think it looks nice. Technically, you can write whatever you like there, including any arbitrary commands.

• To extend it to N URLs, you have to call the macro recursively with the second argument (which will contain the remaining N-1 space-separated URLs). For some reason, an additional \relax is necessary to make it work, found this out by accident:

  \let\URL\url
  \makeatletter
  \def\url#1{\@URL#1 \@nil}
  \def\@URL#1 #2\@nil{\URL{#1}\ifx\relax#2\relax \else; \url{#2\relax}\fi}
  \makeatother
  

• To handle urls with the percent sign % and with the hash sign # correctly, glean some inspiration from Changing the catcode of _ in one command? and further adjust the definitions as follows:

  \let\URL\url
  \makeatletter
  \def\url{\begingroup \catcode`\%=12\catcode`\#=12\relax\printurl}
  \def\printurl#1{\@URL#1 \@nil\endgroup}
  \def\@URL#1 #2\@nil{\URL{#1}\ifx\relax#2\relax \else; \url{#2\relax}\fi}
  \makeatother