One solution would be to open a new window before clicking the link :
WebDriver driver = new FirefoxDriver();
driver.get("http://stackoverflow.com");
// open a new window
((JavascriptExecutor)driver).executeScript("window.open(window.location.href, '_blank');");
// set the context to the new window
driver.switchTo().window(driver.getWindowHandles().toArray()[1]);
// click the link
driver.findElement(By.linkText("Stack Overflow")).click();
Another solution would be to set the target attribute on the targeted element:
WebDriver driver = new FirefoxDriver();
driver.get("http://stackoverflow.com");
WebElement element = driver.findElement(By.linkText("Stack Overflow"));
// set the target _blank on the link
((JavascriptExecutor)driver).executeScript("arguments[0].target='_blank';", element);
// click the link
element.click();
// set the context to the new window
driver.switchTo().window(driver.getWindowHandles().toArray()[1]);