On Steve Goodman's website there is a VM available for free download that contains an almost fully configured layer-4 load balancer based on Ubuntu and HAProxy. It is very easy to set up and is ideal for test environments.
Unfortunately, load balancing only works for the ClientAccess role and not for the Hub Transport role. Perhaps this will come in the next version?
The preconfigured VM is available for download in Hyper-V and vmWare format.
http://www.stevieg.org/e2010haproxy/
KEMP also offers a 30-day trial version of its virtual load balancer: