I use ULSViewer a lot when debugging SharePoint issues. One of the nicest features of the tool is to view the SharePoint log in real-time (and the fact that is works for both SP2007 and SP2010).
Last time I wanted to use the tool, I found that it was not parsing the SharePoint log in real-time. I was however able to open individual log files.
As it turns out, ULSViewer is using *-????????-?????.log as a filter to parse the logs in real-time. I had some ‘older’ log files in the LOGS folder that were not formatted in the ULS format (from running powershell commands).
Removing these files from the SharePoint Logs folder got ULSViewer real-time parsing working again.