Vyatta Community Edition 2 (VC2) is based on Debian, runs on commodity x86 hardware, includes excellent documentation, and supports numerous enterprise features, including serial T1/E1 cards, VLANs, RIP, and OSPF.
Read about it at LinuxDevices
Touted new features in VC2 include:
Debian GNU/Linux compatibility, said to make VC2 “the most configurable, customizable router/firewall on the planet”
Improved performance — routing protocols will converge substantially faster, including BGP performance improvements of between 70 to 80 percent
Improved serial interface support — VC2 adds support for multi-port T1/E1 and T3 cards (in addition to 1-port T1/E1)
Improved hardware detection and compatibility
Experimental support for multicast routing and IPv6