One more along the line of "slacker" posts....
Some/all of my favorite Firefox extensions (not in any particular order)
ScrapBook - save web pages, notes.
Download Helper - Save YouTube and other Flash (.flv videos).
Yahoo and Gmail Notifier - Notifies you when new messages arrive. Though, it can be distracting if you are an email-addict.
CustomizeGoogle Options - I wonder how people who do not use this option ever have a clean Internet experience.
ScribeFire - Its a blog editor that pops up that makes it easy to post to your blog. Can be great if you think of something worth while and do not wish you spend a few clicks to log into your blog.
Google Notebook - It used to be useful for taking notes and putting links, etc that you can access from other machines. I use it most as a bookmarks tab that I can open in any place. Scrapbook is a better option.
There are a few extensions that are very useful, even for the minimal web-development work I do for non-profits: