We use LogMaster in our enterprise environment to monitor a wide variety of logs and to email us when a certain condition or combination of conditions occur. Our LogMaster server monitors the logs of our other critical servers (MS SQL Server, FirstClass, FileMaker 6 & 7 Servers, Retrospect, etc) and generates emails to the appropriate IT person when it detects log entries that we specify.
Since we started using LogMaster, the developer has added many useful features and improvements, taken our input into consideration, and fixed bugs that we discovered quickly. The customer service has always been extraordinary.
For personal purposes on a single computer, it also comes in handy. For example, the hard drive on a new G5 that had LogMaster on it started failing and logging "I/O Error" in the System.log. S.M.A.R.T. diagnostics said the drive was fine, but it was dying. LogMaster (which had been set up to notify me whenever it detected "error" in the log gave me a heads-up on what was going on, while OS X's only symptom was an increasing slowness.
Minor complaints about LogMaster are that the documentation could use some work (for example, it's not mentioned that you need to set up an Apple Mail.app account for outgoing email to work), and the Preferences window should be resizable (it's fixed at a relatively small size).
LogMaster goes far beyond the (also useful) GeekTool that is mentioned in another entry, at least in log monitoring functionality.
Frank Fenn has done an amazing job with this utility and has developed a tool that we rely on for the health of our enterprise environment. I highly recommend it for a wide variety of log monitoring tasks, both local and enterprise-wide.
Director of Information Technology
Chester County Intermediate Unit