Chrome Developer Tools

Developer tools are one of the best things to happen to mankind after sliced bread. The snapshot is pretty much self-explanatory. I can monitor the traffic, TAT, resources, and what not right within my browser. Also starring a 'Console' option wherefrom I could fire Javascript commands.

While in Chrome, just F12 your way to awesomeness!

Update: Here's an excellent presentation highlighting the key features of Dev Tools.