How can I connect an 8-relay board without using 8 digital pins? I'm looking to connect an 8-relay board to an Arduino Uno. A board like this one. However, I don't really want to use up 8 digital outputs if I don't have to. Is there some intermediate circuitry I can either buy or build that would do this? 

For example, can I connect something to the serial so that sending '3' would result in binary 00000011 and thus turn on relay's 1 and 2?

Alternately, and probably more lazy, is there an extension board that would just give me a pile more digital pins?
A likely solution is to use the Arduino SPI interface and the Arduino SPI library to control a SPI relay board.  Here is a 6 year old thread talking about such a board. And another more recent relay thread where SPI is brought up.