http://www.inlab.de/articles/free-and-open-source-load-balancing-software-and-projects.html
This overview is also maintained at the Xing "Load-Balancing" group and will be updated regularly (see https://www.xing.com/net/prib100ffx/load-balancing/). Please contact us with any comments or update suggestions at info@inlab.de.
Apache: mod_athena
http://code.google.com/p/ath/
Apache: mod_proxy_balancer
http://httpd.apache.org/docs/2.1/mod/mod_proxy_balancer.html
Balance
http://www.inlab.de/balance.html
Core Balance
http://core-balance.sourceforge.net/
Crossroads
http://crossroads.e-tunity.com/
Distributor
http://distributor.sourceforge.net/
Eddie (written in Erlang)
http://eddie.sourceforge.net/
Enhydra Director
http://forge.ow2.org/projects/director
HAProxy
http://haproxy.1wt.eu/
Linux Network Load Balancing
http://lnlb.sourceforge.net/
Linux Virtual Server (LVS)
http://www.linuxvirtualserver.org
Load Balancer Project
http://www.jmcresearch.com/projects/loadbalancer/
nginx
http://nginx.org
Octopus Load Balancer
http://sourceforge.net/projects/octopuslb/
OpenLoBa
http://openloba.sourceforge.net/
Perlbal (implemented in Perl)
http://danga.com/perlbal/
Pound
http://www.apsis.ch/pound/
Riverdrums Load Balancer
http://sourceforge.net/projects/rlb/
varnish
http://www.varnish-cache.org/
XLB HTTP Load Balancer
http://sourceforge.net/projects/xlb/
Zen Load Balancer
http://zenloadbalancer.sourceforge.net/