Wireless Diagnostics
Apple (via Dan Frakes):
Wireless Diagnostics can help you resolve wireless connectivity issues by analyzing the Wi-Fi network your Mac is connected to and providing solutions. Wireless Diagnostics is included with OS X Mountain Lion v10.8.4 and later.
If you can connect to your Wi-Fi router, but are having issues with webpages loading, sending or receiving email, music or video streaming, or downloading, use Wireless Diagnostics. After Wireless Diagnostics has completed an analysis of your Wi-Fi network, it will list any issues it finds and offer some solutions.
