Splunk is great for digging through logs but one of the best tools there is for in-depth analysis is Microsoft's Log Parser tool. It basically lets you run SQL queries on many formats of log data. It's great for ad hoc queries but it is easy to script it to generate nicely formatted charts and reports.
The main thing about Log Parser is that with some creativity you can come up with some very cool stuff. For example:
- Show all users who had failed logins on more than one computer on the network within a one-hour period.
- List the users with the most failed login attempts
- List usernames and IP addresses for all users who have ever logged in to a specific server
- Show all users who have logged in at unusual times or from unusual places
Also I know I probably shouldn't be promoting my own stuff but I really do recommend the "Log Parser Toolkit" book I published which has chapters written by a number of industry experts (including the author of Log Parser) that is a great reference for this stuff. My own copy is very worn down and I still use it constantly.