I found out back in '06 there was a Firefox plugin that did it. But it's not out there in the add-ons area anymore.
You could always cheat with Google:
Are you talking about locating the search box on their site - it's usually at the top right of the screen (sometimes gets covered). If you are asking about searching on a search engine like Google, then yes, there is a way to restrict to specific sites - look under something like Advanced search options and there should be a place to put a site to restrict searches to. For Google it would be "what_you_are_searching_for site:sitetosearch" so "Batman site:lifehacker"
Due to the recent security issues, certain features of the site have been disabled. Among them? Search. But let's be honest: search didn't really work that well anyway. Here's a better way to search for your favorite posts.
Their recommended workarounds are to either use the Google
site: operator, or to add lifehacker to Chrome or Firefox.
If you use Chrome or Opera, it is easy to build a custom (Lifehacker) search provider for those browsers so that you don't even have to use the site: operator every time you want to search something on Lifehacker.