Automattic runs WordPress.com, Akismet, Polldaddy, IntenseDebate, Gravatar, and more. Here are our open positions:
And information about how we work, which is a lot like an open source project:
http://automattic.com/about/how-we-work/
If you apply mention [HN] in the subject it'll make it stand out. We typically do a paid trial project of 2-3 weeks on contract before making a full-time offer.