How to "middle-click" on new macbooks

"Check out BetterTouchTool. It adds gestures to your trackpad or Magic Mouse and there's tons of gestures available to set actions to.
To add the functionality you're looking for choose »Trackpad« from the input dropdown and add »Three finger click« as trigger and »CMD+Click« as assigned action. Now clicking with three fingers will open it in a new tab.

For Safari you'll have to activate »⌘+click opens a link in a new tab«, before it works as intended.

ps. You can also set it to trigger with »Three finger tap«, but I can only get it to trigger consistently when I the »Force tab«, which I for one can't do consistently."

"If you use Firefox, Command-Click will open in a new tab.

Or you can hold two fingers on the trackpad and click; this gives you a right-click. You could then select 'open in new tab' from the context menu. (or get an extension like Tab Mix Plus and make right-click open a new tab)"

"Middle is a single-purpose app for adding the middle click to macOS for Magic Mouse and Apple trackpads. You can pick from a few gestures for each device, or hold the fn key and left click for a middle click. (I'm the developer, and the app is not free)

Multitouch is an app for adding trackpad and Magic Mouse gestures to macOS, and middle clicking is one of the available actions that can be tied to a gesture. (I'm the developer, and the app is not free)"
"Middle is a single-purpose app for adding the middle click to macOS for Magic Mouse and Apple trackpads. You can pick from a few gestures for each device, or hold the fn key and left click for a middle click. (I'm the developer, and the app is not free)

Multitouch is an app for adding trackpad and Magic Mouse gestures to macOS, and middle clicking is one of the available actions that can be tied to a gesture. (I'm the developer, and the app is not free)"

Addressing creating a new tab from a link (with Firefox and Safari, havent tried Opera) using your trackpad and mouse button: you can drag links to tabs to replace the content, or you can drop the link at the end of your tab row to create a new tab.

Hopefully answering Zarg, I've found somewhere else in superuser about MultiClutch (which oddly wasn't cited in this question just yet). It seems like it started on early 2008, while BTT is from late 2009 and actually based on MultiClutch. I believe it can solve your issue, if you haven't found about it yet. I don't have any Leopard installed to actually try it.