What it is
Think of it in terms of the SATs: open source search is to Google what free speech is to censorship. Founders of open source search sites make their code available to other developers to use and, ideally, improve upon. Without the cumbersome barriers of licensing fees or other limitations, open search sites allow for disparate developers to work together to return better search results, faster.

Open source search developed as a direct response to companies such as Google that keep their algorithms chained in tightly protected Silicon Valley dungeons. It's Google's algorithm that ranks its search results and decides which items appear at the coveted top of a search page.

Proponents of open source search say the algorithms of Google and others are akin to an editorial judgment call and insist the public should be apprised of the types of judgments being made.

Who's doing it
Grub (owned by Wikia), Lucene, Nutch, Sphinx

Why you should care
You probably shouldn't. At least not yet. But don't write off open source search engines just because they're a brand new search niche, warns Sullivan, who predicts they may prove valuable in the coming years. Though "there's nothing for a marketer to mess with on open source right now… it's going to change so much," he says.