It's not under open source license.
But it's free to some extent (500 MB per day of indexing).
So if you have up to 500MB per day from any number of machines, splunk will be free.
You are not buying splunk, but how much data will splunk index per day.
And no, i'm not selling this software or nothing
, just had the chance to implement it where i work, and folks who are using it are quite satisfied.