Drupal 6, XAMPP and MAMP
I generally use XAMPP for local development, as I like to keep my dev environment portable and seperate to anything native on the machine. I discovered that Drupal 6 is not compatible with PHP 5.3.x, therefore I needed to either downgrade (not sure how/if I could do that with XAMPP) or use something else. I got round this by installing MAMP, which by default uses different ports to XAMPP, therefore they can be run simultaneously, which is really handy if you need to keep switching projects.
This also means that you end up with 2 mysql servers running, but because the mamp one is on a different port, you can still use the one controlled by xampp.