Topic: Lisp
This month, Red Daly announced cl-spidermonkey, a set of Common Lisp bindings to Mozilla's SpiderMonkey JavaScript implementation.
Not long after that, I learned about Marijn Haverbeke, Alan Pavičić, and Iva Jurišić's CL-JavaScript JS to CL compiler. Apparently it is already faster than SpiderMonkey.
Not content with just having JS in CL, Red Daly also has a version of SLIME that integrates Parenscript to provide things like symbol completion (get it on github: https://github.com/gonzojive/slime).
A little while later another surprising discovery occurred: a certain talented hacker who goes by 3b hacked up a SLIME proxy and a fork of Parenscript that lets you run a SLIME/Parenscript REPL in a browser using WebSockets. Apparently this all happened in a couple of days as part of the The 2010 Lisp Game Design Challenge.
Unrelated but still cool, 3b also wrote a CL to Flash bytecode compiler.