I've been using ThunderBolt AS3 for all of my Flex and Flash projects for a while now an don't think I could live without it. It's great for debugging and also helps troubleshoot issues that only my clients seem to encounter :-)
For those who don't know, ThunderBolt AS3 is a Logger extension for Flash and Flex AS3 projects which uses Firebug and Firefox to show traces and debug information right inside the browser. Usefulness: 10/10.

ThunderBolt AS3 is now also available for AIR, and while I haven't used this latest version yet I have no doubt that it's equally as versatile as the 'old' Flash/Flex version. I highly recommend you give it a try if you are building AIR projects (or use the initial version for your Flash AS3 and Flex work).

ThunderBolt AS3 for Flash/Flex
ThunderBolt AS3 for AIR