Open source and free log analysis and log management tools.

The open source log management tools are:

1. OSSEC (ossec.net)  an open source tool for  analysis of real-time log data from Unix systems, Windows servers and network devices. It includes a set of useful default alerting rules as well as a web-based graphical user interface. This is THE tool to use, if you are starting up your log review program. It even has a book written about it.
   
2. Snare agent (intersectalliance.com/projects/index.html) and ProjectLasso remote collector (sourceforge.net/projects/lassolog) are used to convert Windows Event Logs into syslog, a key component of any log management infrastructure today (at least until Visa/W7 log aggregation tools become mainstream).
   
3. syslog-ng (balabit.com/network-security/syslog-ng/) is a replacement and improvement of classic syslog service - it also has a Windows version that can be used the same way as Snare
   
4. rsyslog (rsyslog.com) is another notable replacement and improvement of syslog service that uses traditional (rather than ng-style) format for syslog.conf configuration files. No Windows version, but it has an associated front-end called phpLogCon
5. Among the somewhat dated tools, Logwatch (logwatch.org), Lire (logreport.org) and LogSurfer (crypt.gen.nz/logsurfer) can all be used to summarize logs into readable reports.
   
6. sec (simple-evcorr.sourceforge.net) can be used for correlating logs, even though most people will likely find OSSEC correlation a bit easier to use
7. LogHound (ristov.users.sourceforge.net/loghound) and slct (ristov.users.sourceforge.net/slct) are more "research-grade" tools, that are still very useful for going thru a large pool of barely-structured log data.
8. Log2timeline (log2timeline.net/) is a useful tool for investigative review of logs; it can create a timeline view out of raw log data.
   
9. LogZilla (aka php-syslog-ng) (code.google.com/p/php-syslog-ng) is a simple PHP-based visual front-end for a syslog server to do searches, reports, etc
   

The next list is "an honorable mentions" list which inlcudes logging tools that don't quite fit the definition above:

• Splunk is neither free nor open source, but is has a free version usable for searching up to 500MB of log data per day - think of it as a smart search engine for logs. Splunk includes a tool to extracting parameters out of log data
  
• Offering both fast index searches and parsed data reports, Novell Sentinel Log Manager 25 is not open source, but can be used for free forever as long as your log data volume does not exceed 25 log messages/second (25 EPS). Unlike splunk above, it includes log data parsing for select log formats and thus can be used for running reports out of the box, not just searching
  
• Q1Labs is also neither free nor open source, but is has a free version usable for managing up to 50EPS (roughly 2GB/day). It can be downloaded as a virtual appliance
  
• OSSIM  is not just for logs and also includes OSSEC; it  is an open source SIEM tool and can be used much the same way as commercial Security Information and Event Management tools are used (SIEM use cases)
• Microsoft Log Parser is a handy free tool to cut thru various Windows logs, not just Windows Event Logs. A somewhat similar tool for Windows Event log analysis is Mandiant Highlighter (mandiant.com/products/free_software/highlighter)
  
• Sguil is not a log analysis tools, but a  network security monitoring (NSM) tool, but it uses logs in its analysis.
  
• Loggly now offers free developer accounts (at loggly.com/signup) for their cloud log management service. The volume limitation is 200MB/day and retention time limiation is 7 days. If you'd like to collect and search your logs without running any software, this is for you.