Handle AJAX elements in Selenium 2 (WebDriver)

One approach is to use FluentWait and a Predicate available with Selenium2. The advantage of this approach is that element polling mechanism is configurable. The code example below waits for 1 second and polls for a textarea every 100 milliseconds.

FluentWait<By> fluentWait = new FluentWait<By>(By.tagName("TEXTAREA"));
        fluentWait.pollingEvery(100, TimeUnit.MILLISECONDS);
        fluentWait.withTimeout(1000, TimeUnit.MILLISECONDS);
        fluentWait.until(new Predicate<By>() {
            public boolean apply(By by) {
                try {
                    return browser.findElement(by).isDisplayed();
                } catch (NoSuchElementException ex) {
                    return false;
                }
            }
        });
        browser.findElement(By.tagName("TEXTAREA")).sendKeys("text to enter");

Another approach is to use ExpectedCondition and WebDriverWait strategy.
 The code below waits for 20 seconds or till the element is available, 
whichever is the earliest.

public ExpectedCondition<WebElement> visibilityOfElementLocated(final By by) {
        return new ExpectedCondition<WebElement>() {
          public WebElement apply(WebDriver driver) {
            WebElement element = driver.findElement(by);
            return element.isDisplayed() ? element : null;
          }
        };
      }
      
      public void performSomeAction() {
        ..
        ..
        Wait<WebDriver> wait = new WebDriverWait(driver, 20);
        WebElement element = wait.until(visibilityOfElementLocated(By.tagName("a")));
        ..        
      }