Untangle's technology is, in a word, outstanding. Its Linux-based gateway installs quickly and easily on commodity hardware, providing a graphical management interface for more than a dozen integrated security tools, including a firewall, IPS, anti-malware tools, VPN, and Web filtering tools.
Another Untangle offering runs on Windows XP systems. Unlike the Linux version, this one doesn't require a dedicated PC to serve as a gateway, nor does it require a host system with two network adapters. In fact, it will actually run on a Windows XP system from a USB dongle, yet still provide protection for every PC connected to the same local network.
In theory, Re-Router is designed for very small networks with less than 10 PCs, while the Linux version of Untangle scales easily to networks with hundreds of systems.
Untangle also offers both paid live support and a set of 10 additional gateway applications, including WAN balancing/failover, Active Directory support, and advanced anti-malware tools. These are sold on a subscription basis, including a "Super Bundle" that wraps every paid application into a single discounted package.
Many other vendors play in this space. Very few of them offer the same combination of out-of-box functionality, a strong feature set, and an open-source core product. Fewer still are focused as heavily -- or, in practice, exclusively -- on small-business users.
As CNET's Matt Asay noted in a recent blog post, almost all of Untangle's recent growth is coming from the small-business market. That's a deliberate move on Untangle's part, as is the company's emphasis on developing products that companies with limited in-house IT resources can still put to work.
I use Untangle on my own home network. Along with TrueCrypt, it stands out as an open-source security tool that almost any small business can install, set up, and put to work with very little difficulty.