Thou Shalt Not Confuse Visitors
Some sins feel good, but I don't understand why making hyperlinks open in new windows is so seductive.
This act leaves so many website visitors befuddled ("The back button doesn't work!"), or mistaken ("Pesky pop-up ad!") that it should be avoided. Yet I spend more time than you can imagine explaining this.
That's why I was glad to read in "Small but significant usability sins that websites should never commit" that this no-no tops the list there too:
Don't open links in new browser tabs. Tabbed browsing is for advanced users. If you open a page in a new tab, most users will get lost, start clicking the back button, and then not understand why they can't get back to where they started. Remember that they're not focused on the chrome when they click a link, they're focused on where they're clicking. So it's very easy to miss the fact that a new tab has opened.
The article is worth a read if you aim to put your online project on a righteous path.