I rarely use calculators in sessions but I particularly liked this calculator app that’s available for Mac users as it takes notes of the calculations you carry out.
Available on: Mac OS X
Free version: https://itunes.apple.com/gb/app/calcmadeeasy-free-scientific/id417404127?mt=12
Paid-for version: https://itunes.apple.com/gb/app/calcmadeeasy/id417402227?mt=12