screend, which was originally written by Jeff Mogul at Digital Equipment Corporation and is now maintained by Paul Vixie, is a package that lets you add packet filtering capabilities to the kernel of BSD-based UNIX systems.
Drawbridge, by Texas A&M University, is a package that lets you turn a PC (one that is running MS-DOS and has two Ethernet or two FDDI boards) into an IP packet filter. There are three programs: Filter, Filter Compiler, and Filter Manager. Filter is the program that runs on the PC itself. Filter Compiler and Filter Manager are support programs that run on a UNIX box, and allow you to compile the filter lists into the form needed by the PC, and then download them over the Internet to the PC. (Alternatively, you can transfer them to the PC on floppy, if you can write an MS-DOS floppy disk from your UNIX box.)
The KarlBridge package, by Doug Karl, is a program that runs on a PC with two Ethernet boards. It turns the PC into a packet-filtering bridge.