Logging solution by Pablo Costantini.
Main features:
Multiple output levels (LOG, DEBUG, INFO, WARN, ERROR, FATAL)
Inspection for Object and MovieClip types
Multiple output targets using Publishers (similar to log4j)
Flash 6 & 7 compatible. Flex is also supported.
Small filesize, about 3kb with all available publishers.
GUI Console for viewing and inspecting log messages.
Learn more about it on Pablo's blog entry.
An online example is available.