Highly recommend Soulver in this category as well - I probably use Soulver 100+ times a day, it's particularly awesome around things like throughput - you can throw things like, "10 kbits/second at 30 days in megabytes" - the sort of stuff people screw up all the time with excel when they forget to multiple by 8 somewhere.
It can also do the Currency translation, "1 BTC in USD"
And string a bunch of them, to do things like, "$2/square foot in SGD/square meter"