I've always done most of my programming by myself, whether it was for work or my various open source projects. When I first heard about Extreme Programming and things like pair programming, I pretty much wrote it off as stuff that might be nice for people social enough to program in groups with a greater tolerance for "enterprise" work in general.
Okay, some aspects, like iterative development, seemed pretty cool. Code standards make sense to me, and I try hard to do things like comment my code and use meaningful variable names - in large part because I may have to reuse or modify it months or years later. Stuff like communication and shared ownership of code, though... do I really want to set up multiple personalities talking to myself and arguing about code direction?
The one piece of the XP puzzle that's always been on my "I wish I had energy to do that" list is testing. I've done plenty of testing on all kinds of code in all kinds of circumstances, but it's never been the circumstances I wanted. My testing style hadn't improved much since my ZX81 and AppleSoft days. The kind of code I tend to write is all about XML transformations, with various randomly nested and intermixed structures - not exactly easy unit testing fodder.
Today I finally changed that. I've been building a partial XML parser, and have a whole set of context-tracking structures that I need to have working reliably for the rest to make sense. I knew it worked fine on the particular cases I'd tested, but they were based on complete documents, and didn't necessarily set off every aspect. Writing unit tests (in JUnit) that explore specific aspects of these processes has been pretty easy and extremely useful.
I've already found a few bugs - largely in the way I was testing code before - and feel a lot brighter about the foundations of the code I'm using. In about two hours of test writing, I've reduced the number of places I might search for errors by about a third, and reduced my paranoia about making changes far more drastically. I still have some snarled code ahead of me, but I finally feel like I have the right tools for unsnarling it without create even larger tangles.
I suspect unit testing is probably the piece of XP with the most potential benefit for solo programmers, though CVS has saved me from myself a few times and let me open a wider door to other people's work. Unit testing also combines immediate benefits for me with the prospect of an easier time for other developers who might someday want to build on this work, even if I never hear about it. Seems like a good thing all around.