Selenium is free, but it's worth thousands

First of all, let's point out that "Selenium" is a vague word. There are:

• "Selenium IDE" which is a simple plugin for Firefox, that has limited abilities to record and reply user actions along with using javascript snippets and parameterization.
• "WebDriver , AKA Selenium WD", is a package that can be as libraries to many programming languags such as Java, Ruby, Python and .net framework. Webdriver is way more powerful and flexible because you can use the programming language on your favor; For instance, you might want to use a database to read data as a data driven test, record results to another table, use Java robot to click on non-web elements or desktop windows, or even integrate Webdriver with headless browsers such as PhantomJS, that will help you to run the test in non-gui environments.
• Selenium Remote Control (RC) : This is a server-based selenium, being installed directly on web server and similar to WebDriver, can adopt various programming languages. In my opinion RC is getting obsolete and replaced with WebDriver except in special scenarios such as running multiple Webdriver Agents on client side as a Selenium Grid.

I love selenium because of its vast usefulness and capabilities. In many cases, it is totally compatible with expensive tools such as HP UFT or SmartBear TestComplete and let's not forget that Selenium is free and open source.

However, Selenium and WebDriver are not so strong on their own: They are weak on detecting Java applets and Flash elements and totally out of league when it comes to non-web elements. Fortunately, there are solutions to overcome such issues. For non-web elements, Java Robot or Python gtk is available. For modern Java applets that are based on swing elements, FEST is a useful tool to integrate with selenium to detect applet elements. Finally, Sikuli is an amazing library to locate elements based on their look (pixel by pixel), which is very useful to detect some Flash or Applet elements within GUI.