Drupal modules

• Devel - The development block has links to reset menus and clear cache I use often. The nicely formatted debugging functions like dvr() are nice, but I have been using Zend Debugger more often for things like that (see PDT below).
• Reroute Email - More than once, I have accidentally emailed people from a development site and confused the end users with URLs referencing a localhost domain. This intercepts all properly implemented outgoing mail.
• Coder - Not only does it check for good code formatting, it finds functions you should or shouldn't be using and security flaws if you happen to get sloppy.
• Demonstration site - It was intended for demo sites you could mess up intentionally and then just reset. I find it helpful for testing a change to my local install of a live site that I know is buggy, so I can re-do it over and over without having to reinstall - I just refresh the site from a list of demo snapshots.
• Simpletest - I thought automated testing in Drupal was dumb when I first heard about it, but I confess I have written tests, changed a module, and found out I broke the module by running the test. I do believe I have saved myself from a moderate amount of "D'oh!" moments because of simpletest.

Firefox addons

• Web Developer - CSS Style Information and View Generated Source are my favorite parts of this.
• Firebug - People keep telling me I should have this installed, but in 2 years I haven't found a good use for it, then again I'm not a Javascript fan.
• YSlow - Just informational speed updates when activating some of Drupal core's optimizing through aggregated Javascript and CSS.
• Greasemonkey - I have used this most often for automating checkbox clicking tasks like activating lots of modules after an upgrade.
• split pannel - Useful for a split screen when upgrading views from Views 1 to Views 2.

Development tools

• PDT with the debugger working
• A decent visual diff program. I have been reasonably happy with SmartSynchronize, Meld, and WinMerge depending on your operating system.
• An easy way to submit patches back to module maintainers. I use CrossVC to make patch files against cvs.drupal.org in the proper format.
• #drupal on irc.freenode.net
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