On being wrong in Paris – A great general meditation on the slippery nature of facts, but the specific example is very resonant. We tend to think of places having clear boundaries, but depending on who I was talking to I'd describe my old house as either in "Los Angeles", "Near Thousand Oaks" or "Simi Valley". Technically I wasn't in LA, but the psychological boundaries aren't that neat.
The devil in the daguerrotype details – The detail you can see on this old photograph is amazing, and I love how they delve into the capture method. I was disappointed there was nothing on the role of lenses as a limiting factor on resolution though, I'd love to know more about that.
Katta – A truly distributed version of Lucene, designed for very large data sets. I haven't used it myself yet, but I'm now very curious.
It's cheaper to keep 'em – Your strategy is sometimes pre-determined by what numbers you're paying attention to. If you start off with the assumption your job is to get new users as cheaply and fast as possible, you'll never realize how important retaining existing customers can be.