Spiceworks IT Desktop
Note: This review covers v4.1 of the software.
If you're in a business with a small network--and little or no IT staff--network inventory and management tool Spiceworks IT Desktop may be exactly what you need. It's free and browser-based, and not only provides detailed information about all of your network devices, but also serves as an IT management tool, letting you create and track Help Desk tickets to fix any problems that people have on the network. It has a surprisingly full feature set for a free piece of software.
In addition to doing a network inventory, Spiceworks IT Desktop provides quite a bit of information about each device, such as free space, total disk space, what anti-virus software is being used, and even server connection errors. It also provides an inventory of software installed on each device.
Spiceworks IT Desktop far more difficult to use than Cisco Network Magic, and isn't really suited for home users. And it was not able to properly identify and name all of the devices on my network. Still, you can't argue with free, and it has quite a few useful features.
Note: Spiceworks IT Desktop is free, but includes ads in the interface. If you want to eliminate the ads, you'll have to pay $20 per month. This link downloads the software directly from the vendor's site.