JTAG exists in many processors based on ARM and MIPS core. It is useful for debugging and writing flash.
There are different headers for various chip vendors, I found a good page showing a wide ranges of JTAG header pinouts:
Also, there is a free, powerful JTAG tool for Windows & Linux called "Openocd". It supports ARM7 (ARM7TDMI and ARM720t), ARM9 (ARM920T, ARM922T, ARM926EJ–S, ARM966E–S), XScale (PXA25x, IXP42x) and Cortex-M3 (Stellaris LM3 and ST STM32). You can use it with different JTAG dongles as well. Definitely nice to try it out.