Open Sourced Load Balancer

Linux Virtual Server
The Linux Virtual Server Project is a project to cluster many real servers together into a highly available, high-performance virtual server. The LVS load balancer handles connections from clients and passes them on the the real servers (so-called Layer 4 switching) and can virtualize almost any TCP or UDP service, like HTTP, HTTPS, NNTP, FTP, DNS, ssh, POP3, IMAP4, SMTP, etc. It is fully transparent to the client accessing the virtual service.
Homepage: http://www.LinuxVirtualServer.org/

BalanceNG
BalanceNG is a modern software IP load balancing solution. It is small, fast, and easy to use and setup. It offers session persistence, different distribution methods (Round Robin, Random, Weighted Random, Least Session, Least Bandwidth, Hash, Agent, and Randomized Agent) and a customizable UDP health check agent in source code. It supports VRRP to set up high availability configurations on multiple nodes. It supports SNMP, integrating the BALANCENG-MIB with Net-SNMPD. It implements a very fast in-memory IP-to-location database, allowing powerful location-based server load-balancing.
Homepage:http://www.inlab.de/balanceng/

HAproxy 
HAproxy is a high-performance and highly-robust TCP and HTTP load balancer which provides cookie-based persistence, content-based switching, advanced traffic regulation with surge protection, automatic failover, run-time regex-based header control, Web-based reporting, advanced logging to help trouble-shooting buggy applications and/or networks, and a few other features. Its own event-driven state machine achieves 20,000 hits per second and surpasses GigaEthernet on modern hardware, even with tens of thousands of simultaneous connections.
Homepage:http://haproxy.1wt.eu/

Pen 
Pen is a load balancer for “simple” TCP-based protocols such as HTTP or SMTP. It allows several servers to appear as one to the outside. It automatically detects servers that are down and distributes clients among the available servers. This gives high availability and scalable performance.
Homepage:http://siag.nu/pen/

Crossroads Load Balancer
Crossroads is a daemon running in user space, and features extensive configurability, polling of back ends using wake up calls, status reporting, many algorithms to select the ‘right’ back end for a request (and user-defined algorithms for very special cases), and much more. Crossroads is service-independent: it is usable for any TCP service, such as HTTP(S), SSH, SMTP, and database connections. In the case of HTTP balancing, Crossroads can provide session stickiness for back end processes that need sessions, but aren’t session-aware of other back ends. Crossroads can be run as a stand-alone daemon or via inetd.
Homepage:http://crossroads.e-tunity.com/

balance 
Balance is a simple but powerful generic TCP proxy with round-robin load balancing and failover mechanisms. Its behavior can be controlled at runtime using a simple command line syntax. Balance supports IPv6 on the listening side, which makes it a very useful tool for IPv6 migration of IPv4 only services and servers.
Homepage:http://www.inlab.de/balance.html

Distributor load balancer
Distributor is a software TCP load balancer. Like other load balancers, it accepts connections and distributes them to an array of back end servers. It is compatible with any standard TCP protocol (HTTP, LDAP, IMAP, etc.) and is also IPv6 compatible. It has many unique and advanced features and a high-performance architecture.
Homepage:http://distributor.sourceforge.net/

Pure Load Balancer
Pure Load Balancer is a high-performance software load balancer for the HTTP and SMTP protocols. It uses an asynchronous non-forking/non-blocking model, and provides fail-over abilities. When a backend server goes down, it automatically removes it from the server pool, and tries to bring it back to life later. Pure Load Balancer has full IPv6 support and works on OpenBSD, NetBSD, FreeBSD and Linux.
Homepage:http://plb.sunsite.dk/

Load Balancer Project
The Load Balancer Project is a tool that allows you to balance requests using clusters of servers. The goal is to achieve high availability load balancing with a simple configuration for the load balancer and the network topology. It leaves the servers untouched so the configuration only resides on the load balancer, and it allows you to manage any type of service via a plugin model design and a transparent proxy feature.
Homepage:http://www.jmcresearch.com/projects/loadbalancer/

mod_athena 
mod_athena is an Apache-based application load balancer for large systems. It allows the HTTP server to act as a load balancer either internally to Apache’s own mod_proxy (for reverse proxying), or externally to machines querying it. Arbitrary statistics are sent to the engine via a simple GET plus query-string interface, from which it will then make decisions based on chosen algorithms.
Homepage:http://ath.sourceforge.net/

udpbalancer 
Udpbalancer is a reverse proxy that sorts UDP requests from your clients to your servers. It may operate in round-robin, volume balance, and load balance modes.
Homepage:http://dev.acts.hu/udpbalancer/

MultiLoad 
MultiLoad is a load balancer that redirects HTTP requests to pre-defined servers/locations. It gives the provider a way to balance the traffic and hides the real download location. It allows you to manage different version of each download. It is also a load balancing server extension. You can distribute files on some servers so that a downloaded file can be loaded form different servers. These servers can have different priorities to control the active traffic.
Homepage:http://download.laukien.com