I’ve mentioned Soulver more than a few times on this site and I am going to go ahead and mention it again, because it is an amazing tool.
I have mentioned it before. I use it as calculator, scratch pad and place to keep random numbers at times. They also make a iPhone and iPad app that syncs with each other via iCloud. Doing some numbers on your iPad or iPhone at lunch and get back to yoru desk in the afternoon--the numbers are already there.