We've been using the Linux Virtual Server project (which a previous poster
mentioned) to do load balancing (locally) on web based apps, pop3, smtp and
now iptable firewalls. It scales well, has multiple lb algorithms (wlc, rr,
lc, wrr, etc.) and even multicasts out the connection info if you want so you
can set up a bank of redundant lb's. It can work w/ NAT or ip tunneling or
direct routing so it might be able to do global load balancing depending on
the setup. We have a 10000+ user mail farm behind one and haven't seen a
single hiccup...
http://www.linuxvirtualserver.org/