Although there are actually many applications out there that can do this, it depends mainly on how much work you wish to do, each time you want to monitor a new log on any server.
If you wish to monitor logs on several hosts, the easiest way to do that is with Nagios. Nagios has a tool called LOGROBOT which you can use. This tool is pretty straightforward. Say you have a log file or log fileS to monitor on hosts named x,y,z... fine. Just specify the host names and the absolute path of each log file. Then give it the strings you want to monitor. There's nothing for you to learn how to do. No mumbo-jumbo documentation for you to have to decipher.
See the Nagios Exchange:
The configuration (enabling, disabling, adding of new log checks on new hosts), can all be done from one central server. The central server will be the server on which Nagios is installed. Whenever the strings you specify are found in the monitored logs, email alerts will be generated and sent to whoever you want them sent to. No false positives.