Chip FTDI FT232R has a special mode named “synchronous bit bang”. It could be used for implementing fast serial interfaces like SPI or JTAG. Bitbang mode is described in application note "Bit Bang Modes for the FT232R and FT245R".

There exist several starter boards based on FT232R, for example MMUSB232RL or FT232RL Breakout.

Using these boards, one could easily build a simple JTAG programmer, for example:

Example of openocd.cfg for Olimex SAM7-EX256 board:

# Daemon configuration
#~~~~~~~~~~~~~~~~~~~~~
telnet_port 2001
gdb_port 2000

# Interface
#~~~~~~~~~~
# We use homebrew usbjtag-compatible schematics
# based on FT232R Breakout board from SparkFun.com.
interface ft232r
ft232r_device_desc "FT232R USB UART"
ft232r_vid_pid 0x0403 0x6001

# Target configuration
#~~~~~~~~~~~~~~~~~~~~~
source [find target/sam7x256.cfg]

Patch for OpenOCD: debugger is available here: openocd-ft232r.pch.gz.