Slightly OT: Best JTAG interface?

Hi all.

Sorry if this has been covered here before, but there is too much noise in my searches. I’m in the market for an inexpensive JTAG interface that supports most if not all ARM devices, and is compatible with open source tools in an *NIX environment. I’m only interested USB based devices. No parallel port available. Does such a device exist? If not, what is the best compromise?