Figuring out the JTAG pinout on a device turns out to be the most time consuming hardware portion of many hacks. [hunz] started a project called JTAG Finder to automatically detect the JTAG pinouts on arbitrary devices using an 8bit AVR ATmega16/32L microcontroller. Check out the slides (PDF) from the talk as they break down how one finds JTAG ports on an arbitrary device, with or without a pinout detection tool. [hunz] is looking for people to pick up the project where he left off.
Once you determine the correct pinout, you will need a JTAG cable: there are two main types, buffered and unbuffered, both of which I have soldered up and tested from these circuit diagrams (image of completed buffered cable here). The software most hardware people use today are the openwince JTAG Tools. To get the JTAG Tools to compile, grab the latest source directly from their CVS repository.
The last time we featured JTAG was with regards to Linksys devices, but the tools listed above can be applied to any device with JTAG.