Widely-used load balancing solutions?

I've used Linux VS as load balancer for a 20 GB/day mail system and it worked just fantastic. It will scale for a lot of solutions you may encounter.

An number of free and open source load balancers/web proxies can be made to run on Windows, but configuration is a bit of of pain. I have tried nginx, lighttpd, and goold old Apache for various load balancing setups.

That said Windows NLB is fine, you just need a lot of custom scripting to handle failure scenarios properly. Personally, we use nginx running on Linux VMs in front of our Windows machines, because like most other open source tools, nginx works much better on Linux than Windows.

Pound, HAproxy, and LVS are also used extensively in the open source world.