Every time I make use of the openness of open source, I come to appreciate it more. It's far more flexible than closed-source applications, and lends itself to diagnosis and customization. The other day, I upgraded PHP from 4.3.1 to 4.3.2 to fix some security problems. After that for some reason, whenever I hit a page written in PHP, it would return an error. Since this was something that changed just recently, Google was completely useless. Were it not for the source being available, I'd have been stuck... forced to redo the way PHP is used on my site or to downgrade back to 4.3.1. However, I simply grepped the source code for the error message and found out why it was occurring. With that knowledge in mind, I decided it'd be simpler to just edit the PHP source code and recompile than to redo the way my site uses PHP. A simple fix for a simple problem. If PHP was developed as a closed-source app, I'd have been out of luck for weeks, if not forever (or at least until I redid the pages on my site).
[They've since fixed the problem and I've gone back to using the official sources.]