I like also the PCF8574.
Yes, its not the cheapest.
But very easy to use and expandable (up to 16 on the same 2 lines).

Especially for keyboard debouncing you can use the
interrupt output. It was activated on every changing of
the input state in respect to the last reading.
So only if it was set, you must read the device, to get the new key
state. And if it was not set again, after the debouncing time,
the key state was right.
So you have only very little CPU time consumed.

To access I2C devices:

http://www.specs.de/~danni/appl/hard/i2c/index.htm
http://www.specs.de/~danni/ap.../index.htm


Peter