When I click on a link to an external site that’s been posted on this forum, it starts loading in the current tab, replacing this forum. This is almost never my intent. I think forums function much more smoothly when links to external sites load in new tabs (new windows), rather than the current tab.
What about if you right-click on the link and then choose “open in new tab” or “open in new window”?
Robert Rothman
To provide a counter-point, I (almost) always prefer sites to never open new windows (or tabs) unless I explicitly ask for it.
Ditto for me (with an exception for links in web apps, such as Google Reader). I have no interest in the OP’s proposal.
Not sure how many systems support this, but I simply middle-click to open a link in a new tab. This requires no opening of any menus and no shortcuts.
That approach requires both a three-button mouse and a middle finger. Not everybody has both of those. 
Robert Rothman
They must be very frustrated drivers.
(Note though that you use your index finger to middle-click, not the middle finger - that one is only for right-clicking and communicating with other drivers.)
Ah, having never used a three-button mouse I was unaware of the protocol. I do, however, have experience in communicating with other drivers–although I understand that in some states it is illegal to engage in digital communication while driving.
Robert Rothman
You do, maybe. 
I don’t have a three-button mouse (or, I do, but usually I am just using my trackpad), and I routinely use command-click to open things in a new tab. I believe shift-click opens in a new window.
It probably depends on the browser; I use command-click all the time, but in my current version of Safari (5.1.5) shift-click saves to the reading list, which is a handy little feature which has really helped me cut down my tab bloat. Unfortunately, to open in a new window I have to command-option-click, and even then it doesn’t shift focus to the new window. There’s also the “accidentally option-click to save” feature, which annoys me.