1. Command-L (⌘L) : Put focus on the location bar.  The location bar is where you type URLs.  If I want to do a (Google) search instead of a URL, I often press tab to get to the search engine text box.  ⌘K does that also, but I often find myself pressing ⌘L before I classify my question as a URL or as a search term.

2. Command-T (⌘T) and Command-Shift-T (⇧⌘T) : The first one creates a new tab, the second one restores the most-recently-closed tab in your history.  The nice thing about restoring a tab is it comes back with its own history, so you can use the “back” button if you want to trace your steps.

3. Apostrophe (’) : Search for a link on the page.  This is a recent discovery that has made navigation quite easy.  Instead of using using the mouse for link-clicking, I’ve begun using the apostrophe.  The slash (/) key searches links AND text.  If a link is matched, I press Enter to go there (or ⌘-Enter to open the link in a new tab).

4. Option-Number (⌘1, ⌘2, ⌘3 … etc.) : Move directly to the first, second, third etc. tab.  I save a lot of time this way since flipping between tabs is so efficient.  I try to keep the number of open tabs below 12 or so, and the limited number of single-digits for this shortcut key is what motivates me to do so (aside from CPU and memory limitations!)

5. Option-Space (⌥-Space) : Ubiquity!  I use this Firefox plugin more and more these days.  After pressing the key combo, I type “email this to chris” for example and it wraps up the URL of my current page and sends me off to gmail where I can email something to my brother.  Or I recently created the download-files command for Ubiquity that lets me download all of the images on a page, for example.  Or search-domain-name to see if a domain name is available… or twitter to update my status… or … you get the idea.  Ubiquity can help you leverage almost anything that’s available online.

Note for windows users: in many cases, where you see a ⌘, you can use the “ctrl” key instead.  I’m not a windows guy though, so don’t quote me on it.