O'Reilly Network has reposted an article, called Making Sense of Programmable Logic, that provides a nice introduction to programmable logic. It covers the blurring line between hardware and software and gives an overview that includes CPLDs, FPGAs, VHDL, and Verilog. Since the article was originally published in 1999, a whole Open Source hardware design community has evolved that includes the Free Model Foundry, OpenCores.org, and Open Collector, which provides release announcements for programmable logic and other Open Source Hardware.