The LARTC-HOWTO describes many advanced ways of routing and shaping traffic. Using the HOWTO, you will be able to shape traffic in myriad ways, which will help you lower latency for interactive use when others are downloading or uploading. It will also teach you how to limit certain hosts/protocols' bandwidth usage. It also explains many aspects of routing using the 'ip' tool from the iproute2 suite.
The Seattle Firewall is an ipchains firewall that supports IP masquerading and can be used on a standalone system, on a dedicated firewall system, or on a multi-use gateway/server. It supports VPN via IPIP tunnels, IPSec, and PPTP. It is easily configurable by editing configuration files, and can be extended without modifying the base product. It also includes realtime monitoring with an audible alarm that sounds when suspect packets are detected.
gbrctl is a GNOME utility to configure the Linux Ethernet Bridging code as found in Linux kernels version 2.3.47 and higher, and patched 2.2.x kernels. It features fast and easy bridge and port editing and a stack window, displaying images of your active bridges with 'LEDs' displaying the current port statuses.