When you boot up os x and the login screen appears you can find out some information regarding your Mac.
Right below where it says 'Mac OS X" and it shows your mac's name, you can click on that area and each click reveals different information.
Click it and it will reveal which OS X version you are using.
Click again and you will see what build of OS X you are running.
Click again and it shows your Mac's Serial #.
Click again and it reveals your IP address.
Click again and it shows network account.
Once last click and it shows the current date and time.
(Thanks to Corey from The MacCore podcast for this tip!)