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Kemp have released a free load balancing virtual appliance. If you have a small environment and don't need the high availability of two load balancers, then this could be an ideal solution. 

Announcement: http://kemptechnologies.com/blog/announcing-new-free-loadmaster-application-load-balancer/

Download site: http://freeloadbalancer.com/ 

There are some limitations, particularly around the throughput (only 20mps) but if you are using a small environment or a lab, then it could be all that you need. Absolutely no reason to use Windows Network Load Balancing any more. 

If you have Kemp load balancers in your production environment, then it is an ideal way to have the same in your test environment. It will also make this a valuable learning tool for Exchange and server administrators. 

No support included, but that is to be expected. 

It looks like it is pretty much the complete feature-set from Kemp, including:

Layer 4/7 load balancing

Content switching

Caching, compression engine

MS Exchange 2010/2013 optimized

Pre-configured virtual service templates

The only thing it is missing features wise is Active/Hot Standby redundant operation.

If you are going to use this in a production site, then I would watch that maximum throughput though.