Except for the pluggable USB8Bit board, this is among the simplest DDS-type boards which can possibly be built: Basically, it's just an AVR RISC microcontroller connected to a DA converter.
http://www.triplespark.net/elec/oscillator/PoorManDDS/
A very simple version of the DDS synth, using just the 2313 and a resistor network. This does not use chip from Analog Device. It's controlled over RS232 from a small Windows program, and can generate Sine-, Sawtooth-, Trangle- and Sqare-waves ranging from 0.07 Hz to about 200-300 kHz in 0.07 Hz steps.
http://www.myplace.nu/avr/minidds/