I just read Vidar Hokstad's post on reducing coupling via unit tests, and I can't agree more. I'm currently trying to untangle some code that was written without unit tests, and is very tightly coupled. It's easier to fix in Python than it would be in, say, Java, but that still doesn't make it fun. A recent post on proggit makes the argument that unit testing is on the wane. I really hope he's wrong. I've been doing TDD for several years now, (since reading Kent Beck's book on the subject), and I find it much better than any other methodology.
I was also sad to hear that Agitar went under. I was never in their target market, but I always thought they had some cool tools.