Favorite communication chips (taken mostly from Raffi Krikorian's lecture on 12/1/03):
RS-232 : MAX202 or MAX203
MAX203 is prefered because no external capacitors are needed, but is more expensive
RS-485 : LTC1481
Hardware USART : many PIC MCUs including the PIC16F876 have built-in USARTs
If a hardware USART is not provided, one can buy an external peripheral to do the job or use "bit banging" to simulate the USART. This "bit banging" means that you use software to time data transmission to fool the device you're communicating to into thinking there is a real serial port there. One should rely on a compiler to do this work.
USB : http://www.dlpdesign.com/usb/ have USB ICs.
They have boards with parallel and serial interfaces that convert to USB.
Radio : "Don't build a radio!" - R. Krikorian : instead use these chips
Nordic VLSI nRF401 : Single chip 433MHZ, 200kbps @ 100 meters
Texas Instruments TRF6900 : Mostly single chip 900MHz, 38.4 kbps @100 meters
rfPIC Microcontrollers : PICs with FM transmitters
Also check out chipcon, National Semiconductor, and Cypress. Cypress semiconductor is rumored to have cheap radio chips.
Infra-red : Don't rebuild IrDA unless you are doing something really simple
Simple solution: just hook an IR transceiver to a USART
Complex (or at least not simple): MAX3120 - Single chip IRDA modulator: hook one end to USART and the other end to IR transceiver, 115 kbps @ 1 meter
Wireless LAN (802.11b) : This can be a 'heroic' task
'Spoof' a PCMCIA bus and wire an Orinoco Gold card. This saves you from making sure that the hardware is correct
IOSoft 802.11b development kit for the PIC www.iosoft.co.uk/wlan2.php