Uses the I2C bus to give 16 general IO ports. Multiple chips can be combined to give something like 256 inputs / outputs from the Raspberry Pi.
If using breadboard or vero board you are lookinh gor the "E SP" variety. The others are designed for mounting on the surface of circuit boards.
The chip can be fed from 5v or 3.3v depending on your outputs. There is a thread on Raspberry Pi forum saying if you need it to run at 5v you need to power the chip at 3.3v and use a level shifter.
Resources - http://hertaville.com/2013/04/01/interfacing-an-i2c-gpio-expander-mcp23017-to-the-raspberry-pi-using-c/
| MCP23017 DIP SOP Bidirectional 16 Bit I O Expander I2C IIC Serial Interface | |
 |
$1.57 End Date: Thursday Dec-27-2018 6:01:37 PST Buy It Now for only: $1.57 |
| MCP23017 I2C IIC SPI Serial Interface 5V LCD 1602 2004 12864 Expander Module | |
 |
$2.49 End Date: Wednesday Dec-12-2018 5:18:48 PST Buy It Now for only: $2.49 |
| MCP23017 DIP SOP Bidirectional 16 Bit I O Expander I2C IIC Serial Interface | |
 |
$1.65 End Date: Sunday Dec-23-2018 6:16:02 PST Buy It Now for only: $1.65 |
| 4PCS Microchip Corporation MCP23017 E SP MCP23017 16 Bit I O Expander with S | |
 |
$10.99 End Date: Friday Dec-21-2018 21:16:04 PST Buy It Now for only: $10.99 |
| MCP23017 E SP MCP23017 16 Bit I O Expander Microchip Corporation | |
 |
$3.00 End Date: Saturday Jan-5-2019 17:45:53 PST Buy It Now for only: $3.00 |
| MCP23017 5V 1602 2004 12864 LCD Expander Module I2C IIC SPI Serial Interface | |
 |
$0.87 End Date: Monday Dec-17-2018 4:10:25 PST Buy It Now for only: $0.87 |
| 1PCS MCP23017 DIP SOP Bidirectional 16 Bit I O Expander I2C IIC Serial Interface | |
 |
$1.59 End Date: Tuesday Dec-18-2018 21:29:30 PST Buy It Now for only: $1.59 |
| US Stock 5pcs MCP23017 E SP DIP28 16 Bit I O Expander with I2C Interface IC | |
 |
$13.10 End Date: Saturday Jan-5-2019 22:00:42 PST Buy It Now for only: $13.10 |
| 2PCS Microchip Corporation MCP23017 E SP MCP23017 16 Bit I O Expander | |
 |
$3.91 End Date: Saturday Jan-5-2019 13:56:38 PST Buy It Now for only: $3.91 |
| Microchip Corporation MCP23017 E SP MCP23017 16 Bit I O Expander with S 3 pcs | |
 |
$6.75 End Date: Wednesday Jan-2-2019 16:49:00 PST Buy It Now for only: $6.75 |