Microsoft has experimentally introduced “SearchTogether,” which allows people to use a browser plug-in (IE7 only) to literally collaborate on search. You need a Windows Live ID and Messenger, but you can apparently use any search engine you like.
Here’s a tutorial and some screens.
I haven’t used it so I can’t report on the experience. Conceptually, however, I could imagine that in many use cases (e.g., Travel and Local Search) this could be highly valuable as an option. Microsoft Messenger has had collaboration capabilities around local search and Live Maps for some time, but they’re little used.
Indeed, collaboration is widely available in various online applications; however, it has yet to come to search. The key to mainstreaming this or gaining decent consumer adoption is bundling the option into the browser and not having several “hoops” for users to jump through.
This is very early and will undoubtedly evolve.