MAC OSX 10.8.2 ..which I believe is Mountain lion.
I like it a lot..admittedly, I don't know much about computers, but it works very well for me and my school work. Everything is fast and runs smoothly.
The update was only $20 from the last one (snow leopard I believe..)
Most impressive was the new icloud features, everything syncs up really well. Oh and the messages.. It puts everything I get/send to a person (messages, email, pictures) all in one spot, even if they are from/to my phone, ipad, computer. Which is mighty cool.