Why are external domains appearing in my apache logs?

The probes are by crackers looking for open proxies: some (badly configured) reverse proxies will connect to any domain, not just the domain they're supposed to be serving. They are trying to use your server to connect to and abuse another site.

Anyone can connect to a webserver and request any url they wish from any host. It'll then turn up in your log. An example,

$ nc www.whateveryourdomainishere.com 80
GET / HTTP/1.1
host: www.asdfasdfasdfsdafsdf.com

Will get you an entry in your apache log for www.asdfasdfasdfsdafsdf.com