You can find it for free at http://nginx.org/ I find nginx is simpler to setup than pound (it's not hard but I found it unintuitive and annoying), it seemed to make some basic setups overly complicated with the config file syntax.

nginx on the other hand is perfectly suited in everyway, it is even simpler to setup and seems to be the most stable and most efficient any load balancer.  I would go as far as to say that a good nginx setup is more reliable, faster and more efficient than a hardware based setup, since you can use commodity hardware.

It's also great (and I read that Fastmail.fm is responsible) that they paid the developer to implement a reverse mail proxy feature as well.

What really sold me is how Wordpress is using it to load balance their network, and they posted almost 'scientific' documentation showing the difference nginx made on their servers and how much the load was reduced.

In my opinion I'd say nginx is definitely the best load balancer out there.