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 Real-time graphs
You can create visualizations that update in real time.
OS X, Linux & Windows are supported. You can also access Splunk from iOS and Android devices.
Pro Free version
With restrictions on log size and devices a free version is available.
Splunk can turn searches into alerts.
Pro Understands data in any format or language
Pro Extendable via a large selection of apps
Additional functionality can be added with over 500 apps.
Pro Scales effortlessly
It is built for enterprise use, meaning it scales easily.
Splunk is pretty expensive compared to other solutions.
The interface and service are very antiquated