CryptDB - A PostGres modification that stores data in a securely encrypted way, but still allows efficient queries. I love the idea, can't wait to see the software.
Perceived density - We very rarely go back and look at what the numbers we're using actually mean. This author noticed that the standard population density figures for cities didn't match up with anyone's real experiences with those places. He then reasoned about why that was, created a different way of measuring and built figures that seem a lot closer to reality. New York really is denser than Los Angeles!
Reverse Social Engineering (pdf) - If attackers can persuade their targets to approach them, the attack is more likely to succeed. The friend recommendation systems of social networks like Facebook can be gamed to manipulate those targets into friending attackers. Via Prasanna
If This Then That - Wonderfully simple scripting system for joining together web services like Flickr, Facebook and Twitter.
Hector - A battle-tested Java interface to Cassandra with a strong community behind it.