Double-clicking the empty space in the Tab Bar just to the right of an open tab in Safari will create and open a new tab. Not sure how new this is, but I can't believe I have never thought to try this method before!
This is an addition to the other well-known ways of opening new tabs:
- Command + T.
- Control + Click (or Right Clicking) the empty space to the right of your tab(s) and selecting 'New Tab.'
- Clicking the small '+' sign on the far right of the Tab Bar.
- Control + Click (or Right Clicking) a URL, and selecting the 'Open in New Tab' option.
- Command + Click a URL.
[crarko adds: I've seen all the other methods mentioned, but this one is new to me, too. It also worked in Firefox, and even in Terminal, but not in Chrome as far as I could tell. Post results for other browsers in the comments if you like.]

