Splunk (and the cloud-compatible Splunk Storm) are wonderful indexers of logging data and allow easy search, alerting, and graphical dashboard creation.
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Pro Easy to use, powerful search
It's human readable, intuitive, fast and with auto-complete to boot. And if you need more advanced functionality out of your queries, 120+ page search manual will give you an insight in how much is actually possible.
Pro Widely used
De facto standard for log aggregation, monitoring, analysis and reporting.
Pro Free version
With restrictions on log size and devices a free version is available.
Pro Scales effortlessly
It is built for enterprise use, meaning it scales easily.
Pro Real-time graphs
You can create visualizations that update in real time.
Pro Extendable via a large selection of apps
Pro Understands data in any format or language
OS X, Linux & Windows are supported. You can also access Splunk from iOS and Android devices.
Splunk can turn searches into alerts.
Splunk is pretty expensive compared to other solutions.