I have been doing roll outs of Spiceworks at our company and we are finding it to be a great tool not just for monitoring servers but everything else on the network.
It does things like automatic inventory and custom monitoring to send you emails when there is a problem (EG: Printer is down to 10% of ink or hard drive of this server has 20%).
Its downside would probably be is density of information per computer, dontdon't get it wongwrong it has ALOTA LOT of data per machine but for things like servers where you might want alota lot of stats you might need to use another tool.
EDIT: oh did i mention its business model is based around it being free forever.