Lexi-Comp has a free trial, at least for Android phones. It lasts 30 days. I just bought the Lexi Clinical suite b/c they sent me a 50% off coupon. I got the three year package.
Another resource is the Clinician's Pocket Drug Reference - which has a 7 day trial and then only costs about $10 I think.
I am not crazy about Epocrates, but I think that's b/c a lot of my preceptors don't like it.
I just downloaded the iGeneric app for Android. It's free and tells you what generic alternatives exist for most brand name drugs. I also get the Pharmacist's Letter on my Android phone. That's free if you subscribe the regular Pharmacist Letter.
What other apps do people use??