Hackers are trying to get control to your blog all the time, so it is very important to take necessary actions to prevent your blog from being attacked by hackers. In one of our previous post we have you how to add CAPTCHA in WordPress registration or login form by using Better WordPress reCAPTCHA plugin. If you are looking for more simpler plugin just to add CAPTCHA in login form then you can try Captcha on Login.
Another awesome feature is it also allow to change the default admin username from admin to whatever you want. It is really a good idea to change the username of admin to prevent brute force attacks and turn your blog more secure. at first get the plugin from above link and upon installation you will see the menu Captcha on login. Now set the options:
Here is output in log-in form: