Free monitoring and inventory software like ms system center configuration manager? I know about spiceworks but there you have to log in with account and I don't know if they are sending some of our information online.
You could probably do with some more detail on what you actually need, SCCM is a big beast.
You might have better luck searching for CMDB software...
I have setup http://www.virtu-al.net/2009/10/26/workstation-server-audit-script-v3/ this in the past.. basic but very useful. It could be modified to output data to a database for you to report from.
Depending on exactly what you want the software to do, you might want to check out OpenNMS. It has inventory tools, but also manages/monitors your environment.
Nagios doesn't really have the inventory features, and you can't scan your network (out of the box), but perhaps it's something you want to check out as well.
Not free, but is cheaper than full SCCM, have you looked at System Centre Essentials?
It's a cut-down version of SCCM (and a couple of other System Centre components) that is designed for smaller businesses that don't need/can't afford the full-blown System Centre components.
I'm quite keen on OneCMDB (onecmdb.org), for its easily extensible schema. For monitoring, Nagios is just awesome for its simplicity, and versatility - even if the configuration is awkward..