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"
You can solve all three of those with Wolfram Alpha too, with no need for apps. Seems like Google knows how to solve the last two, but not the first one. Perhaps with some different syntax it would be possible too.
What I like about this app is that there's context and history. I use Spotlight all the time for these conversions, but there's no way for me to keep the last calculations around for reference or reuse; I have to put stuff in a text editor for that, which is a lot more awkward.
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"