They are Y-Combinator companies looking to hire. From the FAQ (link at the bottom of the page): Can I post a job ad? [...] The other kind of job ad is reser

They are Y-Combinator companies looking to hire. From the FAQ (link at the bottom of the page): Can I post a job ad? [...] The other kind of job ad is reserved for YC-funded startups. These appear on the front page, but are not stories: they have no vote arrows, points, or comments. They begin part-way down, then fall steadily, and only one should be on the front page at a time.