There are a number of different chips available that allow you to expand your I/O, both digital and analog.
For digital, common ones include the 74595 shift register as well as the MCP23008 or MCP23017 chip which does does both digital input and output. For analog there are chips like the 74HC4067, or you can use a multiplexor as described here.
The 74595 already has a Netduino.Foundation driver, and the MCP23008 driver is in progress here.
In fact, we're working on a really cool Unified GPIO Architecture that will allow you to use the expansion chips as if they were part of the Netduino itself. For example, the following code creates a relay on an MCP23008 chip:
// create our MCP23008
MCP23008 mcp = new MCP23008(39); // all address pins pulled high
// create a digital output port from that mcp
DigitalOutputPort relayPort = mcp.CreateOutputPort(1, false);
// create a new relay using that digital output port
Relay relay = new Relay(relayPort);
// toggle the relay
That work is in the
MCP23008 branch of Netduino.Foundation if you're interested in looking at it.