Logstash, a fully free and fully open source tool for managing events and logs, let's you collect logs, parse them, and store them for later use (like, for searching). Speaking of searching, logstash comes with a web interface for searching and drilling into all of your logs.
Logstash can easily parse text-based logs, you can query for more precise things like, all 404 http errors, nagios critical alerts in hard state, or mail server faults - all without accidentally finding logs with the word ‘404’ or ‘critical’ in the wrong place.
For an overview of logstash and why you would use it, you should watch the presentation Jordan Sissel gave at CarolinaCon 2011. This presentation covers logstash, how you can use it, some alternatives, logging best practices and parsing tools.
