It's a hybrid engine. I do my own crawling and indexing in addition to relying on external APIs. For the APIs I use, I've built a lot of relevancy and other logic on top of them so they often don't look the same as if you'd query them directly.

I started out exclusively with my own stuff, and then started using BOSS when it came out so I could focus on what I think are my value-adds. Now I use a bunch of stuff (including my own). My approach is to just be a better search engine. If an API does something awesome, e.g. WolframAlpha or embed.ly, I figure why not use it.