Firefox

When you deploy your application online you actually unable to debug it properly, hard to see messages and any other outputs, except Alerts. Luckily I've found Firefox extension which can help me a lot. FlashTracer is project of Alessandro Crugnola, http://www.sephiroth.it. You can download it from Firefox Addons website or directly from its home website - http://www.sephiroth.it/firefox/flashtracer/.
FlashTracer is very intuitive and easy to set up.