Two options come to mind:
1. Use a network drive. You connect it to the router and it can be accessed from all PC's (with the right settings). Here's just one I found with google (it's even wireless), but there are much more. But I'm afraid it doesn't list ps3 as a compatible OS for the systems using it.
2. Use a full-blown fileserver. That's a separate computer in your home network. An example is Microsofts Windows Home server (http://www.microsoft.com/windows/products/winfamily/windowshomeserver/default.mspx), but you can use a Linux box also, in principle. Here again, I expect the ps3 to be the problem. You might need to configure your server as a web-server (as far as I know, the ps3 has Internet access).
Just 2 Windows PC's won't be a problem.
I would like to share a 500gb external hard drive between two computers. I would also like to share it with my ps3, view jpgs and listen to mp3s. Is it possible to do this? I dont like to connect the hard drive to one computer and then make it accessable through a network because it is too slow. Is there a way to do this? What do I need to buy? My wireless router currently died so I need to buy a new router anyways.