Versiera, of course, pops to mind as a competitor, as does FiveRuns (I still don't have the complete scoop on FiveRuns, but check out these screenshots). So far, Spiceworks looks the slickest. Interestingly, both FiveRuns and Spiceworks are funded by Austin Ventures. The conspiracy theorist in me is saying that Spiceworks and FiveRuns could be put on a "sliding scale of management," as we used to call it at BMC.
Quick-and-Easy
Their target is companies of 250 people (or devices) or less. Spiceworks doesn't currently do any management, it just monitors for things like software installed, anti-virus software usage, disk space available, the up/down status of services (Windows and otherwise), the up/down status of devices, and a couple other "there or not there" checks.
While they were explicit about being for "small business" -- not even the Medium in SMB -- that limitation is more a matter of scale leanness. Scaling limitations aside (the ceiling was officially 250 devices, and anecdotally 350-500), after adding a few missing features -- esp. sending emails/pages and historic reporting -- they'd work quite well for any IT shop that just cares about uptime monitoring.
The fact that they're free and install extremely quickly compared to other systems management platforms makes them well worth looking into if you manage any IT, even your home network. I installed it on Kim's ThinkPad, and after opening up the web app's port in the Windows firewall, I had could access it in Firefox on the PowerBook. The out-of-the-box experience is great.
Taking Advantage of Low Barriers to Entry
With such low barriers to entry, it'd seem like an open source or open API approach would help pull in some collaborative development. Currently, their community collaboration with is based on submitting bug reports and feature requests to their forms. They've implemented a "Spicy/Bland" rating on each forum topic so users can vote up or vote down fixing a bug and implementing a feature. That's a great move.
Opening up the platform even more would pull in a whole new channel of collaboration. And, since it's Ruby and Rails based, there's a great community that's into "after hours" coding fun. Indeed, since it is written in Ruby, all of the code is in plain view, so people could start doing that without the blessings of Spiceworks...unless they've obfuscated the code or otherwise locked it down (I didn't check).