Tim Starling from Wikipedia. In his first blog post he talks about the challenges involved in running a website securely where users can upload arbitrary content. It is very cool and very frightening the same time (because it makes you wonder: how many of the web applications out there are verified to this degree). One cool (malicious) trick I didn't know about was running virtual requests in a loop avoid PHP time limitations.
I wish him luck with the blog and hopefully we will see much more of such high quality content there.