I found Spiceworks infuriating when testing. They say it's free but I was constantly bugged to give info, plagued with adverts, etc. I binned it pretty much straight away.
How many servers? If it's a handful, then PRTG Free edition is great. It's very easy to set up and works very well indeed.
Spotlight on Windows is OK but I'm not overly keen on the fancy graphics and also it doesn't report Windows 2008 servers with much accuracy. Eg it will show that 1 million users are connected but only 32mb RAM is in use...
If you know Linux then check out Nagios or Zabbix.