Re: I know there's always Google, but...
They're diversifying, but the core product can be thought of as grep on steroids.
There are shiny product pitches, but basically it collects logfiles from, or runs scripts on anything, chucks it into an index cluster of brobdingnagian proportions and then greps your amalgamated data really really fast. Main use cases are IT Ops and security analytics where the swiss army knife approach to schema-on-the-fly means you can correlate anything with anything.
It's proprietary, licensed in gigabytes per day of indexed data. Lots of places index terrabytes per day: they pay lots of money in licensing and get lots of value out of it by being able to fix stuff before it breaks and costs money.
[Disclaimer, I've got a load of Splunk certs and think it's actually pretty cool]