I discovered Firebug this week via a neighbor helping me with some troubles with the intrawebs. My favorite feature so far is being able to adjust HTML and CSS in the browser without changing the page.
Before I discovered firebug, I made trial-and-error HTML and CSS adjustments to pages and refreshed for everyone to see the mess I made. If a client saw the site refresh online at a bad minute, I was screwed.
Not anymore. I can make adjustments and see what happens before I upload the site.
Another useful tool is being able to ping page content to see what loads slowest.
Firebug is by default available for Firefox and Firebug Lite is available for other browsers AND as a bookmarklet (Drag the link to your bookmark toolbar in order to use it).
Thanks Firebug.