My TV (a recent Philips) has four inputs:
Standard coaxial
RCA/Component
RCA/S-Video
RCA (on the side - all the other inputs are in the back)

In coaxial input I have a coaxial splitter attached where I can choose between A and B inputs. The A input currently has the antenna (I cancelled my cable service because I just wasn't getting my money's worth). The B input currently has nothing, but is for one day when I get an Atari, Colecovision or INTV.

My PS2 is hooked up to the component inputs. If I get an X-Box, Gamecube, or next gen system I'll buy a component switch box.

I have a radio shack S-Video 4-way switchbox hooked up to the S-Video (and stereo RCA) input. The switchbox can take input either from S-Video or RCA video so I have my SNES and N64 (S-Video) hooked up to it as well as my SMS and Genesis (RCA). The switchbox came with blank stickers so I was able to label each button on the switchbox correctly as well as attach labeled stickers to each system' wires.

My NES is currently hooked up to the RCA-only input. If I get another RCA/S-Video system (TG16, hopefully) I'll have to get another switchbox and may just segregate between RCA and S-Video systems.