A while ago we had an add URL page but it was better at catching low quality pages than pages we wanted to index. We are considering ways to enable it again without that issue.
I don't really have any favourites, except Mojeek of course. But I've always used multiple search engines and rarely perform a search on just one.
We have provided our results to multiple metasearch engines over the years and currently provide them to a number of people for various uses. It's not our main focus right now, but could well become as time goes on.
We're at a very early stage with our new ad platform and we have a select number of testers helping us to improve it. So far, we're very happy with the interest that's been shown in it. Our current financials are good and I've tried to only move onto the next stages when I felt it was achievable, so right now we're stable.
We originally seeded it with URLs from DMOZ. Now we have our own page weight that in combination with modified dates decides what is crawled and when.