Keeping with the famous science teacher saying of "every number must have a unit," Equivalence (iTunes link) is the one-stop converter for all things units. Did we say "one-stop?" Why yes we did, because Equivalence can convert between over 260 units in 31 categories.
Once of the nice features of Equivalence is the ability for the application to auto update the unit conversions over the air, instead of through the App Store. So, if the developer decides to add a unit, or if a unit were to change, updates could get pushed out to the application.
The beauty of this application is when you flip your iPhone into landscape mode. Just like the built-in iPhone calculator, this application adds much more functionality when horizontal, including a calculator that can add units together.
Bottom line: This is the best unit converter for the iPhone, hands-down. Be sure to take a look at our gallery of Equivalence screenshots. You can buy Equivalence from the iTunes App Store for $4.99 (US).
Update: The developer has dropped us an email to let us know about a special promotion. Equivalence will sell on the App Store for $1.99 until October 6th. The pricing changes have already been submitted to Apple for approval and should appear soon. Also, we have added two new screenshots in the gallery showing off the landscape mode in Equivalence.