
Additionally, let's say a patient describes a "little round blue pill". I can use the pill lookup tool to help identify what that medication is by entering information such as the size, color, coating, or imprint. Obviously, the more information you have about the pill, the fewer potential matches you'll get.
Here is a screen shot from Epocrates.
Another of my favorite apps is Mediquations Medical Calculator. This one currently has 190 medical formulas and scores. I can change the settings to use either US or SI units. Many of the formulas and equations are also referenced, so I can look to see how various formulas were developed or the intended use. Also, I set up a list of my favorite equations; the ones I use most often with students. That way there's no scrolling around looking for the ones I use every day. Each also shows the complete equation so I can use it as a teaching tool.

Here is a screen shot from Mediquations showing how to change the settings to use either SI or US units for each equation.
There are many equations that would be of use for RDs, including Basal Energy Expenditure, Corrected Calcium, conversion of HbA1c to average blood glucose, Ranson criteria for scoring acute pancreatitis, and stool osmotic gap.
Another cool option is to email the results of an equation to a colleague.
I've also just added MedRef to my phone. MedRef was designed with the medical student in mind but I'm finding it to be incredibly useful when teaching students in the clinical setting, as well as to refresh my memory on normal lab values. There is a small reference section that will be incredibly helpful once additional information is added. MedRef might replace all those little notes that we carry around!
I also use PubMed Plus for the iPhone and am thinking of adding a couple more medical apps. I'll update when I do!