I've had good luck with specifically the WinTV card and any kind of regular video camera. It works great under linux, and wasn't that hard to set up. xawtv comes with some command line utilities (one called "snap" will grab a photo whever you request it), and if you're handy with the PHP, it would be very easy to hook snap up to some scripting, and have it all cached and controllable by a web-page.
Fluffy has a great idea, in using some specially designed security camera hardware to complement the linux software. xawtv's snap will allow you to select different tv-channels that you want to snap from, so if you have 4 cameras, (going into a "modulator", i believe) one on channel 54, one on 55, 56, and 57. Plug that modulator into the TV card, and you could very easily grab a single frame from any channel at any time, and post it to anywhere on the internet you wanted to.
Hardware requirements for grabbing frames are a -little- more strenuous than a 486. Performing straight raw-pixel frame-grabbing on my p200 mmx is sometimes a little slow. Don't worry about doing "live streaming stuff". That's just dumb, IMHO, unless you can dedicate a lot of bandwidth, a lot of time, and don't have a tv-station to broadcast on.
You'll have to do a lot more research than this in order to get a good idea about what products will meet your needs, but this should be a pretty good start. Good luck, and you'll have to tell k5 what you did, so that we can come down and visit! ;^)=
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