[WSS]WSS/MOSS ULS log and how to read and write it

Eli Robillard has given a good blog entry introducing the WSS/MOSS ULS log which is important for sharepoint troubleshooting:

#SharePoint Trace Logs and the Unified Logging Service (ULS)

Also, there is code sample in MSDN about how to write custom provider that helps write custom messages into ULS log:

#Trace Log Example