The PI uses 3,3V levels on all its GPIO pins.
before I go on a little warning:
Do not try to put hard (that is able do deliver lots of current) 5V levels on the PI, as you might succeed in lifting the PI's 3V3 supply up to 5V through the protection diodes embedded in GPIO pins, and putting 5V on the 3V3 line can destroy chips on the PI! So when you put a 5V signal on a PI GPIO pin, at least put a 1 series resistor in between the 5V driver and the GPIO pin, or better yet, divide the 5V signal to 3V with a resistor divider consisting of a 2K2 and a 3K3 resistor.
But as the propeller is also a 3V3 powered chip it can be directly interfaced to a PI without any problems! Digital signal levels are completely compatible.
The Raspberry PI has many hardware assisted I/O capabilities, maybe the most obvious way to connect a raspberry PI and a Propeller is to use the UART, but do note that the boot software of the PI has assigned the UART port to a command terminal, if you want to use it for your own purposes you should program Linux to release the UART port before you can use it.
Obviously SPI and I2C are other candidates, or you can use multiple parallel ports and built your own bit banged interface with them.