I need some more GPIO for my application. What should I do?
The most suggested way is to use some kind of port expander, which usually communicates over some serial interface. Pavel Bánský suggested some over SPI, like MAX7301 or MAX7317 by Maxim, Tony suggested MCP23017 (SPI/I²C) by Microchip.
How could I drive high current outputs / isolate the processor's pins?
One part worth mentioning is the 6N139 optocoupler (several manufacturers), which can take as low as 0.5mA and output as much as 60mA while providing optical isolation, and is therefore suitable for directly connecting to the processor's pins. You can then for example wire AC high-voltage relays to it. However, if you are going to control DC motors, you should check out the L239D (L239) dual bridge (also by several manufacturers), which is designed for this task, and can deliver up to 600mA (1A).