General

Bodycalc
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 eg iPhone4S 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. Detailed review of Datecalc

Drugs and Diseases Search (phones) , Drugs and Diseases Search HD (for Tablets)
Quickly search for drugs or diseases from free resources: drugs.com, medscape, epocrates and wikipedia. The app retains your last search term and makes it easy to search for the same item from multiple sources.

Pubmed mobile
Search Pubmed with the U.S. National Library of Medicine (NLM)’s mobile friendly version of PubMed.gov

Shots
Shots by STFM is a quick reference guide to the Childhood, Adolescent, and Adult Immunization Schedules for the United States. (Homepage)