I had no clue. Mr. Gruber did:

It’s not widely publicized, but those integrated search bars in web browser toolbars are revenue generators. When you do a Google search from Safari’s toolbar, Google pays Apple a portion of the ad revenue from the resulting page. (Ever notice the “client=safari” string in the URL query?)

The same goes for Mozilla (and, I presume, just about every other mainstream browser.) According to this report by Ryan Naraine, for example, the Mozilla Foundation earned over $50 million in search engine ad revenue in 2005, mostly from Google.

Heck, if they can read my gmail to decide what advertisements to pimp in my face, I guess I can't act like this is a big deal. Still, is there a capitalism-free crevice anywhere nowadays? If open source software doesn't have it... This is part of the reason I'm starting to like RMS all the more.