Most browsers that support Javascript also support Netscape-style proxy autoconfiguration (PAC) files (I'm not aware of a JavaScript-capable browser released this century that doesn't). PAC files contain JavaScript code that is executed to determine what proxy (if any) to use for each request.
function FindProxyForURL(url, host) {
if (shExpMatch(host, "*.example.com")) {
return "DIRECT";
} else if (shExpMatch(host, "somewhere.else")) {
return "PROXY someproxy:8080";
} else {
return "PROXY default-proxy.example.com:3128";
}
}
To have a single setting for browsers and other applications that don't support Javascript, you'll need a proxy that supports per-URL parent proxies.
Squid is one possibility, it's a caching proxy designed for high loads and with many features. See Squid selects parent depending on requested URL? for examples of how to set up per-URL parent proxies.
Wwwoffle is another possibility. This proxy is strongly oriented towards having an offline cache, and caches more aggressively than is allowed by the HTTP standards, which is a problem with some sites. Parent proxies can be specified per-URL with wildcard patterns (the default configuration files contains commented-out examples).
