I know there is going to be a lot of opinions of the multitude of medical apps you can put on your palm/crackberry/iphone.
One of the most important ones is epocrates. It's a drug guide. It lists all the pharmaceuticals you will be prescribing in your near future. It's got adult/peds dosing, mechanism of action, costs, drug to drug interactions, etc. etc. etc. I use this program on a daily basis.
Oh...and it's FREE.
It will be more useful during your clinical rotations, but it will be nice to plug in some drugs our profs will be mentioning during lectures.
www.epocrates.com
I wouldn't pay for the upgraded applications. The free ones will do the trick.