I have personally downloaded and tested (on an iPhone only) the following applications for the iPhone, iPad, Blackberry, and Droid.  Not everyone would benefit from these apps, as some of them are tailored for specific subject areas.  Also, I recommend that clients make an informed decision before investing in these apps by reviewing information on apps and customer reviews.  It is important to note that self-help materials such as these apps are not an appropriate substitute for psychotherapy and/or medication.  However, when combined with an effective treatment regimen, these apps may be helpful resources and tools.

Apps for Clients

1,000 Exercises (iPad only-$5.99)
101 Doses of Recovery for addictions/recovery ($0.99)
Body-Mass Index (BMI) Calculator for weight management/nutrition (free)
CBT Referee for therapy self-help ($4.99)
eCBT Mood for mood monitoring ($0.99)
Gratitude Rock for positive affirmations ($1.99) 
iCBT for therapy self-help (iPad only-$7.99)
iCouch CBT for therapy self-help ($1.99)
iDrink Smart ($0.99)  This app is ONLY for clients with a goal of responsible drinking.  I would discourage clients from using this app without first consulting with me on its use.
Men’s Health Magazine for helath and wellness info (iPad only-free)  
Men’s Health Workouts for fitness ($1.99)
My Food-A-Pedia for weight management/nutrition (free)
My Thoughts for self-monitoring and journaling ($1.99)
mym3 for mood monitoring ($2.99)
STAT Depression Screener (free)
StepsAway for locating recovery meetings (free)

Apps for Mental Health Professionals

3D Brain (free)
CodeDSM ($0.99) 
DocGuide (free)  
iDrink Smart ($0.99; may be helpful with substance abuse evaluations by calculating approximate BALs)
MedAlert, a.k.a. "Medical Breakthroughs" (free)
Medline Plus Mobile (free)
MedPage (free)
Medscape (free)
Mental Health News Reader ($0.99) 
MerckMedicus (free)
PLoS Medicine (free) 
PsychTerms (free)
Psychiatry On Call ($4.99)