War on Legacy Code

Following the hallowed US American tradition of declaring war on whatever things you don't like, I've decided to declare war on legacy code this week.
By legacy code, I mostly mean untested code, following the definition of Michael Feathers' book Working with Legacy Code. That's a great read, by the way. If you're working with old code, you should go read it. I found that I've been doing most of the things mentioned already, but it's nice to see a systematic write-up about it.
My chosen battlefield in this war is the code at my day job, mostly because it's in a much worse shape than the code I deal with in the various Open Source projects I'm involved in. Seeing how my day job code is a Frankenstein's monster of Perl, PHP and Python parts, some of the work will be to get some of the tests done twice. In particular, I really want to get rid of the PHP parts.
I won't delve into the particulars of the code too much, it's published under the GPLv3 if anyone is interested. I will however try to post some daily news from the front lines, with things that I have thought about during that particular day of the battle.