This application enables you to quickly perform calculations on Body surface area, Body Mass Index and Ideal Body Weight in one little package. It also allows you to quickly work out drug doses by BSA or Body weight.
Datecalc (non-iOS5) Datecalc2 (iOS5 only)
While not strictly a medical app, I find it very useful to be able to work out what day falls X number of days or weeks ahead when one is scheduling appointments. It is also quite handy when you want to know how many days have elapsed since surgery or transplant for example.
Commonly employed Performance scales/scores used in oncology: Karnofsy score, ECOG (also known as WHO or Zubrod scores), Lansky scores and a handy ECOG-Karnofsky converter.