Microsoft LogParser and LogParser Studio

Search through very large files for relevant text. When Notepad and Notepad++ can't handle it, LogParser easily does the job. 

You can get the tool here: LogParser

You can get the GUI LogParser Studio here: LogParser Studio

Sample Queries

Commandline: 

LOGPARSER “Select Text from C:\Filemon.log where Text like ‘%Access Denied%'” -i:TEXTLINE -q:Off

Studio: (first select logfile type)

SELECT TEXT FROM '[LOGFILEPATH]' where TEXT like '[text to search for]'