Ben Brooks over at The Brooks Review making a great case for Soulver on the mac:

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.

Simple and powerful, cannot beat that. Get it on their site or the Mac OS App store.