Selenium - Functional Test Automation.

Key Selenium benefits on a glance,

1. Platform independent test scripts (OS Supported).

2. Browser independent test scripts (Browser Supported).

3. Language independent test script (Language Supported)

4. Natural Support for AJAX driven applications, which very few tools provide.

5. Tools around it, interesting was Bromine(Web Based Test tool for selenium, natural integration).

6. Very active community (Wiki, Jira and releases).

7. Good list of committers & supporters.

Reference
http://seleniumhq.org/