It really depends on your needs, and your vSphere license levels. I'm mostly wondering if you have the option of leveraging vCenter (both native functionality and the APIs).
Quite a few vendors make free versions of their performance monitoring products (Veeam ONE, VMturbo, vFoglight, vKernel vOPS, SolarWinds, etc). As you probably know, VMware also provides vCOPS Foundation free with all vSphere editions. Is there something in particular you don't like about vCOPS Foundation, that you're hoping to address with another free solution?
-Andy
VCAP5-DCA, VCP-DV 4/5, MCSE, space camp graduate.