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It's not a webpart - but here is a tool that I'm writing to help administrators diagnose problems with their sharepoint farm.
http://www.codeplex.com/sharepointloggingspy
Have you tried Excel? Just right click the log file > select "open with" > choose excel. It opens in a tabular format. You can then add auto filters to slice and dice your data. This does mean you need access to the filesystem of the SharePoint server or to create a share for the logs folder.
Hope this helps
Dave
I open mine in Excel2007 - right click on the file and select open with and then select Excel
Don't open the log that is being written to at that time - make a copy.
Andy, I knew I'd find it if I kept hunting around..
This is a feature that plugs into your Central Administratio website. I haven't tried this yet, so not sure how good it is, however it seems to offer filtering capabilities that should help in debugging/troubleshooting.
http://blogs.msdn.com/chrisfie/archive/2007/05/15/analyzing-unified-logging-service-uls-logs-using-sharepoint-s-central-admin.aspx
Paul.
There is a great log file viewer:
http://www.codeplex.com/wssmosslogfilereader