On my old Treo, I had a great conversion program which I used frequently. When I switched to the iPhone 3G, I was hoping to find a similar program. The best I have found so far is "Units".
Units is a conversion program that allows you to convert currencies, length, area, temperature, energy units, volume, time, speed, and weight. It has a simple, straightforward interface. First you select the mode you want to convert (say, currency), then the "from" units, then the "to" units. You select these options be repeatedly tapping on the "mode", "from", and "to" buttons until you get the selection you want. Then, using the calculator style interface, you type in the number you want converted. While I haven't used the currency converter yet, the currency values are updated daily (automatically) or you can update them yourself manually.
The program also comes with three "quick launch" buttons that are customizable, so you can choose the three conversion types you do most often (including setting the default "from" and "to" units). In addition, it also has a "switch" button on the main screen, so you can quickly switch the "to" and "from" options.
A secondary screen gives you a little more control. You can choose the mode and units from a list, rather than repeatedly tapping on a button to find the one you want. In addition, this is also where you can customize the shortcut buttons.
All in all, I found this to be the best conversion program available so far. The interface is clean and easy to use, and it offers a wide array of conversion options. I do with it had a greater number of units to choose from. For example, while it does list liters under volume, it doesn't list microliters or mililiters. Some of the units it does offer are useful and unexpected - for example, under temperature it includes kelvin (most programs simply have F and C). Eventually I also hope to see a greater number of units to choose from, and potentially more modes (for example, a shoe size mode would be great when travelling abroad).
At $0.99, this program is priced right,and well worth the cost for anyone who will find themselves in need of converting units on a regular basis.
Pros: Cheap, simply interface, easy to figure out
Cons: Repeatedly tapping to choose mode or units can be tiresome, some modes a lacking some units
All in all: 4 out of 5 stars.
Please note, this is the program from Crossroad Solutions. There is another program named "Units" from Ben Spratling. I didn't find this one as easy to use.