As already stated : HAProxy (Score 5, Informative)

If you're looking for a lightwheight open source loadbalancer with a lot of features, go for HAProxy. In my company we work with F5 Big IPs, Alteon, Cisco CSS which are the leading load balancers from the industry, they are really expensive and depending on the licence you buy, you won't have all the features (HTTP level load balancing, cookie insertion/rewriting). We first used HAProxy for POC and now we're installing it in production environnements, works like a charm on a linux box (debian and RHEL5) with around 600 users.