I use Marc Liyanage’s PHP5 distribution on my Mac instead of the version that comes installed with OS X; sooner or later I usually find that there’s some extension that’s either missing or too old for a particular development requirement, and Marc’s distribution is usually a little more up to date and complete.

File this under "It goes without saying," but this one had me so puzzled for a time that I thought it bore repeating.

In general, a good best practice for any Drupal site is to put all third-party modules under your sites/all/modules directory; this makes for good portability when it comes time to upgrade the Drupal core, and makes it generally easier to keep track of what you've installed.