By default elasticsearch runs assuming a one machine, one node setup (You specify node data in elasticsearch/config/elasticsearch.yml), so what happens if you want to run multiple nodes on one box, say, you want to play with multiple nodes on your dev machine?
The easy answer is that you could create multiple elasticsearch.yml files (elasticsearch.0.yml, elasticsearch.1.yml etc etc) and then start each instance from the command line referencing the new config files.
usr/local/bin/elasticsearch -fD es.config=/usr/local/Cellar/elasticsearch/0.xx.x/config/elasticsearch.0.yml
usr/local/bin/elasticsearch -fD es.config=/usr/local/Cellar/elasticsearch/0.xx.x/config/elasticsearch.1.yml
That should get you most of the way there (the new node comes up on port 9201), but if you have any problems and need an alternative read this detailed response on Stackoverflow